Ouch. Rough stuff for those in-state students applying to in-state schools. (Out-of-state students pay more money...) from the Washington Post.

Labels: nova, politics, virginia

Yesterday, we had a joint NOVA-University of Pittsburgh at Johnstown field trip to Shenandoah National Park. It was a great day of examining new rock outcrops and old treasured favorites. UPJ is responsible for one of the only departmental student-centered geology blogs that I am aware of, Mountain Cat Geology. A couple of weeks ago, igneous and metamorphic petrology professor Elli Goecke contacted me about local rock options, and I invited her crew to team up with the NOVA GOL 135 field course to check out Shenandoah. [Geoblogger small world: Elli studyed under Kim Hannula in Vermont!]

Together, the sixteen of us checked out evidence for the two Wilson Cycles recorded in the rocks of Virginia's Blue Ridge province, and had a pleasant time hiking around and enjoying unparalleled fine weather. Unfortunately, November means the days are short, and we had the sun set on us before we got to the final stop (at Signal Knob Overlook). We took a group photo there: see if you can spot who's a NOVA person and who's a UPJ person...


The annotated version, to show who's who:


Thanks for a great day in the field, everyone!

Labels: blogs, blue ridge, field trips, national parks, nova, pennsylvania, shenandoah, virginia

Richard Dawkins is on a speaking tour in promotion of his new book on the evidence for evolution, which I just got yesterday. He's not coming to DC, but the closest speeches he'll be giving are in Charlottesville, at UVA (Oct.16), and then in Philadelphia, at the library (Oct.22).

Labels: evolution, meetings, pennsylvania, virginia

George Mason Univerity invites you to attend the first lecture of this season's Vision Series on Monday, September 21, 7:00 PM, in the Center for the Arts. Jagadish Shukla, Distinguished Professor of Atmospheric Oceanic and Earth Sciences, will present "Global Warming: Science, Adaptation and Mitigation". A reception with light refreshments will follow the lecture. The Vision Series lectures are free and open to the public. Tickets are available online, or at the CFA ticket office, or the evening of the presentation. For more information and to reserve tickets, click here.

Labels: climate change, global warming, virginia

This is another photo from Saturday's hike. Unakite is rumored to be the 'state rock' of Virginia, though it's not in the state code. Regardless of its official status, it sure is a distinctive sight: An epidotized granitoid, unakite is identified by the distinctive pairing of pistachio-green epidote and pink potassium feldspar. There's some grey/purple quartz there too. In the mid-Atlantic states, it's only found in the Blue Ridge geologic province. Here, on the trail below Dark Hollow Falls in Shenandoah National Park, my friends and I encountered this lovely boulder of unakite bearing a vein of milky quartz.

The original granitoid was Grenvillian in age, about 1.1 billion years old. Presumably the metamorphism took place during Alleghanian mountain-building, between 300-250 million years ago. Unakite has been quarried in Virginia for use as a building stone, and can be seen as tiles on the first terrace of the steps leading from the National Mall up to the southern doors of the Smithsonian's National Museum of Natural History in Washington, DC.

Labels: blue ridge, dc, igneous, metamorphism, minerals, museums, smithsonian, virginia

First Geological Society of Washington meeting of the new academic year
September 23
http://www.gswweb.org/
Dupont Circle, Washington, DC

Virginia Region of the National Speleological Society caving convention (but without the caving)
September 25-27
http://www.varegion.org/var/events/FallVAR/FallVAR.shtml
Battle of Cedar Creek Campground (Route 11, between Strasburg and Middletown, Virginia)

New York State Geological Association meeting
September 25-27
http://www.newpaltz.edu/geology/nysga.html
New Paltz, NY

Virginia Geological Field Conference
October 2-4
http://web.wm.edu/geology/vgfc/2009.php
Big Meadows, Shenandoah Nat. Park, VA

Labels: caves, gsw, new york, shenandoah, structure, virginia

Virginia geologic map overlay for Google Earth -- you can click on the different units and it will tell you what rock type/formation they are. Pretty cool. Kind of clunky when I loaded it up on my home computer this morning, though.

Hat tip to Kyle House's Geologic Frothings blog for the alert.

Other states available too.

Also worth noting is an interactive Potassium-Argon age date map. In Virginia, you can use it to find the age of the lamprophyre dikes at the upstream end of Mather Gorge (~369 Ma) or find Alleghenian-aged pegmatites, or look at Triassic diabase ages contemporaneous with supercontinent breakup.

Labels: art, geology, google, isotopes, maps, tech, virginia

Two cool courses taught by my esteemed colleagues... If you're local and love rocks, you should enroll in both of these!

Birth of the Appalachians. GOL 135-003A. Saturday, June 20. A one-credit field course to investigate the paleogeography of Virginia, prior to the initial uplift associated with the Appalchian Orogeny. We will be specifically looking at rock outcrops representing the pre- and post-uplift topography and environments, based on evidence in the present Shenandoah Valley of Virginia. Light hiking and roadside geology. Contact Victor Zabielski for more information: vzabielski@nvcc.edu

Mid-Atlantic Field Geology (for educators and interested others). GOL 295-050N. A coherent series of one-day regional field trips, plus on/off-campus lectures and labs on Thursdays (2 - 8:20 PM) during second summer session (also two Saturdays: 7/18, and 8/1). This is an introductory-level, four-credit lecture/lab/field hybrid course, tailored to educators and interested others. It considers local outcroppings of the mid-Atlantic Coastal Plain, Piedmont, Blue Ridge, and Valley and Ridge, as natural classrooms for the demonstration of geologic principles, the study of earth history, and the collection of demonstrative hand-samples. Specific meeting places/times and preparation will be sent via student VCCS email addresses & Blackboard. Class # 12594. If more information is necessary, feel free to email: krasmussen@nvcc.edu

Labels: field trips, geology, nova, virginia

Our third Rockies training hike took place Saturday. Six of us hiked White Oak Canyon, in Shenandoah National Park. It was about an eight mile loop, with six big waterfalls on it. There were a lot of plungepools where other hikers were swimming.

There wasn't an astounding amount of geology on the trail: it was mostly Catoctin Formation, with a few outcrops of underlying Grenvillian granitoids. A few nice amygdules; no columns.

The waterfalls sure were purty, though. Here's Jason at the uppermost falls (86 feet tall):


Me departing from one of the lower falls:

Photo by Chris McMahon

I got home tired and sore from this hike -- it was a good time, but I slept well last night as a result!

Labels: blue ridge, falls, geomorphology, national parks, proterozoic, shenandoah, virginia

As a high school student in Arlington County, Virginia, I used to take regular hikes down a path called Windy Run, and then walk along the south shore of the Potomac River, upstream. It was in the days before I knew anything about rocks, and I was mainly appreciating other aspects of nature, like the plant life, the birds, the bugs, the salamanders, and occasionally something really cool like a raccoon. But I was aware that the scene I observed and enjoyed was not the same scene that had always persisted.

I heard rumors from my uncle about patches of woods inside the DC Beltway that preserved virgin forest -- giant trees that gave a hint of the former majesty of this eastern hardwood forest. I read about an eastern herd of bison that would migrate north and south through the Piedmont and Coastal Plain, crossing the Potomac near Alexandria (before we killed them all). I noticed a gazillion deer, and had it explained to me that the lack of predators like cougars and wolves resulted in the herbivores' population explosion. We used to have elk here, but European colonists had extirpated them. The last of the bison were killed off by 1800, and the final elk met a bullet around 1850. This used to be a pretty wild place!

I observed trash nearly constantly, often mixed obscenely with natural debris, sheathed in mud, or woven into birds' nests. Every few minutes, a jet airplane on its approach to National Airport would thunder overhead. Those of us who lived in the flight path would learn to automatically put conversations on "pause" during the 30 seconds it took for the planes to pass. Visitors didn't know what to do about the noise; it was too pervasive to be ignored. But live here long enough, and you learned to ignore it. You adapted, like the birds adapted by putting aluminum foil and plastic bags into their nests.

And the river itself? It's gross. In the modern day, it's constantly muddy and silty, with a foul-smelling sewage/sediment biofilm all over the rocks and logs in the water. There's scummy flotsam and rumors that you'll get a rash if you swim in it. There's people fishing down by Teddy Roosevelt Island, and you have to wonder why... They're not going to eat the fish they catch out of this polluted stream, are they?

The theme of this month's Accretionary Wedge is "time warp." The Wedge is a geoblog 'carnival,' though it's been inactive for a while, this month sees its return to 'accreting.' For those of you who are new readers to NOVA Geoblog, it's probably a great opportunity to check out some of the dozens of other interesting geoblogs out there. So what does this have to do with my reflections on the local woods, and the Potomac River? This month's Wedge host is Lockwood from Outside the Interzone. He asked geobloggers, "Where and when would you most like to visit to witness and analyze an event in Earth's history?"

I'm going to use my time travel experience to go back in time right here, in Washington, DC. I want to go back to 1491*. I want to see what my home looked like before European settlers showed up and brought their particular brand of industrialization / civilization / land use changes / ecological perturbations to the Potomac River valley. It may surprise readers to learn that I'd opt for this -- a simple experience of pre-colonization North American nature -- over something tectonic and structural, but that's what calls to me on a deep, emotional level. I want to see a vibrant ecosystem with big trees. I want to see the water of the Potomac River look like water; I want to go swimming in it. I want to see what bird migration looked like before it dropped off so precipitously. I want to see a passenger pigeon, a carolina parakeet. I want to see for myself what a healthy amphibian population looks like. And bison fording the Potomac in Alexandria... perhaps emerging from the clear water with the autumn colors ablaze on the far side of the river? That would just be... awesome.

* Note that there's a good book by this same name, on this same theme, 1491. The book makes the case that there was already a lot of landscape/ecological modification playing out before Europeans arrived: that native Americans played a significant role in messing with natural systems and we shouldn't imagine an ecological paradise, just less of an ecological disaster.

Of course, going back to 1491 may have some negative aspects to it: there would be malaria endemic to DC at that time, and the native tribes might not take kindly to a time traveler popping in to ogle their forested homes. But I'll take those risks (they exist today in other places I've visited), since the pay-off would be such a profound deepening of perspective.

If I had the ability to go back in time, I'd use it to gain experience with pre-colonial North America. I'd check out the same river banks I would walk 500+ years later, and see what we've lost.

...And, once I've seen that former world, I can't guarantee that I'd come back.

Labels: amphibians, birdies, blogs, dc, environmental, geologic time, mammals, piedmont, rivers, virginia

And now, a few images from April's Environmental Geology class field trip. We made three stops: (1) a large coal-fired power plant in Maryland, (2) Westmoreland State Park in Virginia to look at coastal erosion, and (3) Prince William Forest Park in Virginia to look at pyrite emplacement and acid mine drainage.

Here's one of the bluffs on the Potomac River at Westmoreland:

Note the recent pile of breakdown in the middle of the bluff where all the water seepage is, and also the orange trail as soil from the uppermost bluff has marked another mass wasting event's passage down to the river.

These are Miocene-aged sedimentary layers known as the Calvert Formation, part of the Coastal Plain. In places, the gray clay has been altered along fracture surfaces, as shown by these orange stripes criss-crossing one another. My toes for scale:


The students spent some time searching for fossils: this is an area where lots of shark teeth are found. We didn't have much luck, but after a long cold winter, it was nice to be standing in the warm sunshine and water:


At Prince William Forest Park, we hiked down to the Cabin Branch Pyrite Mine to look at the massive denudation there due to acid mine drainage, and we also spent some time poking around for treasures, in this case chunks of pyrite:


We had better luck than at Westmoreland...




...But of course we were in a national park at Prince William, so we left the pyrite where we found it. (Westmoreland, in contrast, allows you to keep any fossils you find in loose sediment: that figures, eh?)

I'd like to say that the group of students I had in Environmental Geology this past semester was terrific, one of the best groups I've worked with in a long time. Maybe it was because the class was discussion-focused, or maybe it was the cookies we ate every Tuesday night, but it was a great experience for me, and I'm looking forward to teaching the course again. Thanks, everyone, for making it so much fun!

Labels: coastal plain, environmental, field trips, maryland, miocene, nova, ore, piedmont, virginia

Here's some cool field trips being offered by the Virginia Museum of Natural History. Not particularly cheap, though.

Labels: field trips, fossils, museums, virginia

The Commonwealth of Virginia is searching for a new state geologist.

Hat tip to Lee Allison for this notice!

Labels: geologists, geology, virginia

Today I got the news that I've been accepted to two professional development opportunities that I'm really looking forward to:

1. SERC workshop: Strengthening Your Geoscience Program, June 2009, Williamsburg, Virginia
2. GSA Field Forum: Structure and Neotectonic Evolution of Northern Owens Valley and the Volcanic Tableland, California September 2009, Owens Valley, California

Yay!

Any other geobloggers or geoblog-readers going to be at either?

Labels: california, conferences, virginia

On Wednesday, April 1, the U.S. Geological Survey will kick off a new public lecture series at their national headquarters facility in Reston, Virginia.

The USGS "Science in Action" public lecture series will be held the first Wednesday of every month. These evening events are free to the public and intended for a general audience to familiarize them with science issues that are meaningful to our daily lives. The USGS speakers are selected for their ability and enthusiasm to share their expertise with an audience that may be unfamiliar with the topic.

The first one: From 7-8pm, Dr. John Jones, an expert in remote sensing, will discuss several projects in the Shenandoah National Park and the Everglades. Learn how science from satellites can help decision-makers address issues related to climate change, water resources, and habitat conditions.

Upcoming lectures will be about climate change, hurricanes, energy, glaciers, and a fascinating expedition down the Congo River.

Lectures are held at the USGS building located at 12201 Sunrise Valley Dr., Reston, Va.
For more information on the next three talks - speakers and abstracts - visit the USGS Public Lecture Web site or call 703-648-4748.

Hat tip to Dave Schumaker's Geology News blog.

Labels: meetings, virginia

An article in today's Kids' Post discusses rock collections.

Hat tip to Brian R!

Labels: dc, minerals, teaching, virginia

It's getting to be springtime... and that means field trips!

My first field trip of the semester is tomorrow: my friend David Dantzler has organized a trip to look at stratigraphy and structure out on a new highway in West Virginia. I'm supplying half a dozen Honors students and a NOVA minivan, but David's handling the content. And of course, I'll be on hand to comment on "teachable moments." Looking forward to it.

Other trips upcoming this semester: Billy Goat Trail (x4!), Massanutten Mountain, Old Rag Mountain, Washington DC walking tour, and a weekend-long structural geology trip to the Blue Ridge and Valley & Ridge provinces. I love field trips; really they were the aspect of majoring in geology that appealed to me the most - the fascination with Earth processes took longer to develop.

See you in the field!

Labels: blue ridge, field trips, maryland, nova, piedmont, teaching, valley and ridge, virginia, west virginia

This was forwarded to me today; I'm posting it as a "public service announcement" for local cavers:


Prepared by the Virginia Department of Conservation and Recreation (DCR) Natural Heritage Program in consultation with members of the caving community and staff from the Virginia Department of Game and Inland Fisheries (DGIF). This statement is supported by the Virginia Cave Board, the Virginia Speleological Survey, the Cave Conservancy of the Virginias, the Board of the Virginia Region of the National Speleological Society, and the Butler Cave Conservation Society.

White Nose Syndrome (WNS) is a condition that has resulted in the deaths of hundreds-of-thousands of bats in the northeastern United States since 2006. It is associated with a Geomyces sp. fungus that grows at cold temperatures and colonizes the skin of animals in cold environments. This is consistent with properties predicted for a causative agent of WNS-associated cutaneous infection. More on WNS can found here.

This winter cases of White Nose Syndrome (WNS) have been confirmed in Pendleton County, West Virginia. In February, surveys of significant bat and recreational caves in adjacent Highland and Bath counties in Virginia discovered what appears to be WNS in Breathing Cave in Bath County, located near the Highland County border. On March 3 in response to caver reports of dead bats, a survey of Clover Hollow Cave in Giles County identified a second apparent WNS outbreak. Specimens from both sites have been sent to USGS National Wildlife Health Center in Madison, Wisconsin for analysis.

This winter, 18 Virginia caves in Giles, Bland, Page, Wise, Lee, Bath, and Highland counties have been surveyed for WNS. So far, signs of WNS have only been observed in Bath and Giles counties. However, since Highland County lies directly between Bath County and Pendleton County, West Virginia, along the same belt of karst, it is likely that WNS is present there as well. There is mounting evidence that humans may play a significant role in transmitting biological agent(s) responsible for WNS in bats. The strongest such evidence is that WNS is showing up first in the Virginias in recreational caves with high visitation. It is known that some of those cavers visiting the affected West Virginia Caves had previously visited WNS affected caves in New York. While it is unlikely that humans are the sole or even the primary vector, transmission of the disease by humans may increase both the rate of expansion and geographic extent of WNS.

The Virginia Cave Board and Natural Heritage Karst Program are asking for a moratorium (a voluntary ban) on all underground caving activity in Virginia until April 15, 2009. A new statement will be issued on or before that time. Please note that at that time, we may ask to further extend the moratorium. During the moratorium, three tasks will be accomplished.

1. Scientists from DGIF and DCR will work to identify significant bat caves to which access will be limited in an attempt to slow the spread of WNS by humans. A significant bat cave is defined as a cave used by rare or endangered bat species, significant numbers of common bat species, and/or a high diversity of bat species. Input from cavers is sought and encouraged. A list of these caves will be posted on the VAR List of Closed and Limited Access Caves. Please note that many of the significant bat caves are already gated, closed, and/or have limited (seasonal) access.

2. Additional caves will be visited by biologists to better assess the current extent of WNS in Virginia. We will work to temporarily restrict access to caves in which WNS is present.

3. DGIF and DCR scientists will work with other biologists studying WNS to establish and refine protocols to reduce the possibility of transmission of WNS by cavers. The two major protocols under consideration are listed below. Comment on these protocols is welcome, especially during the moratorium period. Cavers choosing not to observe the moratorium are strongly urged to follow these draft protocols. Once the moratorium is lifted, all cavers will be asked to follow protocols to reduce the risk of transmission of WNS.

Draft protocols to reduce transmission of WNS in Virginia by humans

1. Geographic isolation of caving activity and/or gear: Cavers are requested to limit their caving activity to one caving area as defined by a county or group of counties. The designation of caving areas appears at the end of this document. We are asking this because sterilization procedures are likely not 100% effective, especially since neither the specific causal agent nor mode of transmission has yet been definitively identified. Cavers choosing to cave in more than one of the defined caving areas are asked to dedicate a specific set of clothing and caving gear for each respective caving area. If you must go caving in multiple areas, complete disinfection of vehicles is recommended between trips to different areas. Special care should be taken to segregate any cave clothing and equipment dedicated for use in a given area from all other sets of clothing and equipment dedicated for use in other caving areas.

2. Strict adherence to decontamination procedures when moving between caves (even within the same geographically designated area). After exiting one cave and prior to entering another cave, even within a designated caving area, decontamination practices as outlined by the U.S. Fish and Wildlife Service must be strictly adhered to. ALL equipment must be cleaned! These procedures are posted online here. To reduce the risk of harboring or carrying the disease on their persons, cavers should thoroughly clean and scrub themselves with soap and hot water following each trip to each cave. All clothes worn while traveling to and from the caving area should be laundered as soon as possible following caving trips.

List of Virginia Caving Areas
Alleghany Highlands: Alleghany, Bath and Highland counties
Shenandoah Valley: Frederick, Clarke, Warren, Shenandoah, Rockingham, Augusta, and Page counties
Middle James and Roanoke River basins: Botetourt, Rockbridge, Roanoke, Craig (except RT 42 south of Newcastle) counties
New River North: Giles, Montgomery, and Craig counties (only RT 42 south of Newcastle)
New River South: Bland, Pulaski, and Wythe counties
*Holston: Smyth and Washington counties
*Clinch: Scott, Russell, and Tazewell counties
*Powell: Lee and Wise counties
_________________________________________________________________________________
* - These areas are in the upper Tennessee River basin, and include the northeastern boundary of the range of the Gray Bat (Myotis grisescens). This species is not yet affected by WNS. If you are caving in these areas, please only cave in these areas. If you are not caving in these areas, please do not start.

Labels: caves, disease, mammals, virginia, west virginia

The Commonwealth of Virginia has put up a website soliciting suggestions for the expenditure of the state's share of the federal stimulus package. Got a good idea? Drop them a note.

(The Washington Post reports on a decaying bridge in Arlington County as a good candidate - I drove under this bridge on Tuesday morning. Thursday morning at 4am, softball-sized chunks of concrete fell off the bottom onto Route 50. Yikes.)

Labels: economics, politics, virginia

Labels: granite, metamorphism, piedmont, quartz, structure, virginia

I've discussed the phenomenon of jointing on this blog before, and how when rocks fracture, sometimes they leave behind structures we can see that tell us something about the jointing process. Where did it start? Where did it stop? To answer these questions, we turn to structures like plumose structure, arrest lines (concentric ribs), and hackle fringes.

On this past Sunday's field excursion out to the Massanutten Synclinorium (Shenandoah Valley), MSSE John Graves and I saw some more nice examples of these phenomena, and as usual, I took some photos of them.

Let's start with this one, which shows plumose structure (and thus joint propagation) starting at the right and heading to the left.



A closer-up shot of this same fracture surface (in the Ordovician Martinsburg Formation):



Here's another one (in the Devonian Needmore Formation):



Sorry -- no sense of scale in that (above) one -- it was a few feet above my head. Total width of the photo is about two feet (call it half a meter).

This one (also in the Needmore) shows some really wavy plumes:



At the end of joint surfaces, we find hackle fringes, these "rough edges" where the little ridges and valleys of the plumose "topography" flare up and out in a spiralling kind of shape. When you slice through this spiral shape, it appears as a series of little itty-bitty joints at an angle to the main joint. Here's some hackle fringes on a joint surface from the Martinsburg Formation:



Each of these represents the edge of the fracture at one point. But then stresses built up again past the rock's strength, and it cracked anew, extending the fracture and producing a new hackle fringe. A closer-up shot (rotated) of the above fringes:



And back to the Needmore again, for a lovely series of hackle fringes that I've shown you before, but I couldn't resist photographing again. But to mix it up a bit, this time I used a penny instead of a quarter for scale...



Contrastified version of the above, with annotations:



Lastly, remember that I showed you this photo on Monday, from the Billy Goat Trail?



Well, I think you can see some hackles there, too. Take a closer look...

Below, I've zoomed in on the far upper right of the previous photo, and rotated it 90 degrees. I've also transplanted the penny from another part of the photo to maintain a sense of scale, and drawn a quick sketch of the fractures:



I think the little itty-bitty fractures (again, infused with quartz, making them weather out in high relief) traversing the main left-right joint trace are hackle fringes associated with that joint. Anyone care to differ?

Labels: devonian, geology, joints, ordovician, quartz, structure, virginia

Yesterday I mentioned finding a new (to me) outcrop of the Martinsburg Formation's graded beds (turbidite sequences shed off the late-Ordovician Taconian Orogeny here on the east coast of North America). Today, I'd like to share a few images of where John Graves and I went next: up into the heart of the Massanutten Synclinorium, the Fort Valley. To remind you of the relationship between the Shenandoah and Fort Valleys, here's a Google Map I've posted before:



There, defining the ridges of Massanutten Mountain (and thereby separating the lower Shenandoah Valley from the upper Fort Valley) is the Massanutten Sandstone, a Silurian-aged quartz sandstone (in some places it's a quartz-pebble conglomerate) that is correlated to the Tuscarora Sandstone further west in the Appalachian Mountains' Valley & Ridge province.

The Massanutten can show some nice primary structures, including some of the oldest known terrestrial plant fossils (preserved as fragmentary carbon films) and cross-bedding like this:



With regard to the cross-bedding, note that this is "reverse" cross-bedding, which records shifts in current direction over time. At the bottom of the sample, the current was flowing from left to right, and at the middle and top of the sample, it was flowing in the opposite direction, right to left. This sample shows well the distinctive shape of cross-beds: they are tangential to the main bed at the bottom, but are often truncated on top, making them superb geopetal indicators. (They tell you whether your rock is right-side-up or up-side-down.)

I took John on a hike up the Veatch Gap trail, because I wanted to show him the awesome anticline in the Massanutten Sandstone that NOVA adjunct geology instructor Chris Khourey and I had found on a reconnaissance trip out there in May of last year. John and I took a "group shot" with the fold:



And here's John showing those Montanans that we do actually have some cool geology out on the east coast:



So, what's going on here? Well... the Valley & Ridge province of the mid-Atlantic region is defined by folded (and thrust-faulted) sedimentary strata. These folds were produced about 300 to 250 million years ago, during the Alleghenian phase of Appalachian mountain-building. The tectonic cause of this deformation is interpreted to be North America's collision with Africa, closing the Iapetus Ocean and completing the assembly of the supercontinent Pangea.

More locally, the Shenandoah Valley and Massanutten Mountain are structurally underlain by a great fold, the Massanutten Synclinorium. Synclinoria are different from mere synclines because they are more complicated: the overall synclinal shape is "decorated" with numerous smaller anticlines and synclines. It's a big trough-like shape, but wrinkles are "parasitic" on the main fold. So, even within the big "canoe" shape of the Massanutten Synclinorium, there are little bulges and wrinkles that go the opposite direction. This anticline is one of them.

At that point, having seen the anticline, we weighed whether to keep hiking or not.

We opted to press on... and I'm so glad we did. ... Twenty feet further down the trail, we saw another two anticlines!



At its base, this one had a small cave I could crawl into:



And: a short distance further we found a hiker's shelter with an apt name:



Ha! I love it.

More tomorrow, when I'll revisit the issue of plumose structure and hackle fringes.

Labels: appalachians, geology, mountains, msse, ordovician, primary structures, sediment, silurian, structure, valley and ridge, virginia

For those of you who missed it, here's video of Mike Tidwell's talk at NOVA last Thursday.

Labels: action, climate change, global warming, maryland, nova, podcasts and vodcasts, politics, virginia

Because he's coming to campus tomorrow (Thursday), last weekend I watched Mike Tidwell's movie We are all Smith Islanders. It's a 35-minute long documentary about how climate change is effecting the states of Maryland and Virginia, and the District of Columbia. Though it is a political document (and not a scientific documentary), I think it's a worthwhile enterprise because it connects the global to the local. We hear a lot about climate change, but when someone actually walks through Ocean City, Maryland, pointing out what three feet of sea level rise would look like, it fosters a connection based on shared landmarks.

Thanks to archive.org, you can actually watch the movie in low resolution on the Internet. Google video also keeps a copy available.

Or, if you'd prefer it in higher resolution (on DVD), you can find it at the NOVA library.

Labels: action, climate change, dc, global warming, maryland, movies, nova, politics, virginia

There's a proposal to turn the Luck Stone diabase quarry south of Leesburg into a big reservoir for increasingly-populous Loudoun County, Virginia. It would then be followed by other tapped-out quarries in the area. Collectively storing 8 billion gallons, the reservoirs could serve the surrounding area for up to 120 days during a prolonged dry spell. The idea is to create the reservoirs by siphoning of about 40 million gallons a day from the Potomac River, starting in 2017.

These diabase intrusions are mafic igneous rocks that intruded into the crust during the opening of the Atlantic Ocean. As Pangea broke apart during the Triassic and Jurassic, a huge system of sags opened up in the crust. These low spots were the sites of (a) intense sedimentation, since water flows downhill, and (b) mafic igneous intrusions, since the thinned crust allowed decompression melting of the underlying mantle. (Partial melting of an ultramafic source usually yields a mafic distillate.)

The entire system of failed rift valleys extends along the same trend as the Appalachians, but further east, all the way up to the Bay of Fundy. Collectively, they are called the Newark Supergroup, after one of the larger rift basins in Newark, New Jersey. Dirty sandstones filling that basin were the source of all the 'brown stone' that made the brownstones of New York City. Locally, in our own Culpeper Basin, the main rock that is quarried is diabase, which has a coarser crystal size than basalt, but smaller crystals than a gabbro. It is distinguished by a lot of pyroxene.

Source for the reservoir proposal news: Today's Loudoun Extra, from the Washington Post

Labels: atlantic, culpeper basin, jurassic, minerals, mining, new jersey, new york, news, triassic, virginia, water resources

President Obama is acting to (potentially) improve the fuel efficiency of cars manufactured in the United States. The official announcement will apparently come later today, but somehow the newspapers always find out first.

Meanwhile, Virginia's budget shortfall has led to the elimination of pollution inspectors, which means that instead of the usual inspection of 1400 sites in the Commonwealth this year, the reduced staff will likely get to 800 or so. In Maryland, by contrast, the article describes how the governor is actually increasing funding for the oversight of power plants. An interesting contrast from two Democratic governors (one of whom is now moonlighting as the head of the DNC).

PS - My Prius is less efficient in the cold. I've been hovering between 46.5 and 47.0 mpg for the past couple of weeks. Brrr.

Labels: energy, environmental, maryland, news, oil, prius, virginia

Recently, Andrew Alden compiled a list of state minerals and state rocks. A quirky piece in today's Washington Post explores what Maryland is urging its citizens to do with their state fish: eat them. The story also, somewhat randomly, includes a limerick composed by Virginia's former governor and current senator, Mark Warner:

We have a state dog and a fish and a bird.
And of the fossil I'm sure you have heard.
So why not a bat?
What's wrong with that?
The state beverage is no more absurd.

For some reason, I hear this limerick in my head in Carl Kasell's voice...

Labels: birdies, critters, fish, fossils, mammals, maryland, politics, virginia

Strange Maps has an interesting piece up today about where West Virginia came from (as a state): turns out it was all about the Civil War. The accompanying map shows the original proposed name for West Virginia, "Kanawha," as well as a proposed demarcation between Virginia and Maryland that trended along the western margin of the Blue Ridge physiographic province. If this boundary had come to pass, Virginia would have gotten the Valley & Ridge province, but Maryland would have retained the Blue Ridge, Piedmont and Coastal Plain.

Labels: blogs, blue ridge, coastal plain, history, maps, maryland, piedmont, valley and ridge, virginia, west virginia

According to a study by the Washington Post, our area's electricity consumption dropped by a small but perceptible amount in the first nine months of 2008, as compared to 2007. The article linked to above describes the sources of data as being a mix of home audits in Arlington County, Virginia (40 out of 89,000 total), government figures for the number of miles driven on local roads, and utility billing information. Overall, the Post estimates a 2% reduction for the study period in 2008 as compared to the previous year. Now, the question is, Why? By their reckoning, it's likely to be a mix of increased consciousness of "green" energy practices, increased use of compact fluorescent light bulbs, and perhaps most importantly: mild weather.

One thing I can say about that lattermost factor: this year, 2009, is so far not really of the "mild weather" variety. It's dang cold here!

Labels: dc, energy, environmental, maryland, news, virginia

Yesterday, I mentioned the results of the Virginia DGMR symposium's breakout session on Blue Ridge geology. Today, for completeness' sake, I'll also post the results of the other two breakout sessions (one on the Valley and Ridge province, and one on water issues):

Session 2: Unresolved Issues in Valley and Ridge Geology
In general, the consensus was that continued support for detailed geologic mapping is needed, which provides essential information for a number of major issues that need to be resolved. Major issues discussed during the Breakout Session generally fell into one or more (much overlap) of the following categories:

Geologic Framework:
Need to find better ways to communicate the value of geologic mapping (e.g. beyond the anticipated development along the I-81 Corridor).
Better communication between geologists working in the BR and VR regarding the timing and influence of major tectonic events; presently somewhat disconnected.
Evaluate the influence of basement normal faults.
Better understanding of the sequencing of thrust faults in the VR.
Better understanding of the transition between brittle and ductile fracturing in the VR.
Influence of the Rome Trough on VR structural framework.
Better understand the link between Allegheny Front and BR overthrust.
Better define stratigraphic units with economic and geohazard potential that are currently lumped (e.g. SDu, Ols, etc).
More focus on deep stratigraphy, where there is potential for deep saline aquifers that may provide geologic storage of CO2.

Geologic Hazards:
Identify, evaluate, and prioritize geologic hazards in VR.
Karst/sinkhole digital database; better understanding of the interplay between geologic controls and land use.
Large block slides, recognizing risk areas.
Better understand the nature of brittle on ductile fractures in context of subsidence risk along the western margin of the Blue Ridge
Quaternary history (e.g. evaluation of flood risks)

Economic:
Better understand the distribution of high-Ca limestones; not just limited to the middle-Ordovician units.
Stratigraphic relationships between Devonian shale units (e.g. Millboro, Marcellus)
Evaluate occurrences and distribution of non-polishing aggregate resources.
Evaluate occurrences and distribution of high purity silica (glass sands); (e.g. depositional character of the Oriskany).

Other:
Source and structural implications of Eocene-age igneous rocks (Highland Co) and alkaline dikes (Augusta Co).
Need for improved educational outreach.

Session 3: Integrating Geologic and Hydrologic studies - Benefits and Challenges
The group was divided into geologic information providers, hydrologic information providers, and hydrogeologic information users. The provider groups developed a list of the most important types of information they provide. The User group developed a list of the most important types of information they need. Based on the results of this exercise and the interest of the group, the remainder of the meeting focused on addressing gaps in available information for end users.

Geologic data providers provide:
Geologic formation information - descriptions, structure data, "cover" materials
Fracture/joint data and analysis
Detailed and regional maps
Ground truth on karst features

Hydrologic data providers provide:
Well data - location, yield, logs, geochemistry
Spring data - location, discharge, geochemistry, dye trace results
Water use information - ground water and surface water
Technical /Assistance
Hydrographs

Users of hydrogeologic information need:
Geologic maps at an appropriate scale.
Water well data - yield, depth to water table, etc.
Hydrographs

Users noted the following characteristics of information would be helpful:
Widely available and accessible information (Internet)
Standardized - uniformity of format and methodology (example pumping tests) on at least a statewide basis
Geologic descriptions and maps could include a characterization of saprolite.

Providers and users believed there was good agreement between the type of information that is being provided and that which is needed. The clear problem was recognized to be a lack of data in many places and the lack of easily accessible information (digital format, available on internet)
To address this problem, it was suggested the users of hydrogeologic information may be able to provide some assistance, such as:
Well data - construction characteristics, yield, drawdown, geochemistry over time
Soil data - maps, field notes
GIS Capabilities and data processing - especially county governments.

An identified issue:
Disconnect in some cases between geologists and soil scientists

Next Steps:
Water for America initiative may be an opportunity for geologic and hydrologic data providers to work together at the state level and provide hydrogeologic information to end users.

Based on USGS efforts in Northern Shenandoah Valley, integrated studies have both benefits and challenges. A future meeting to highlight these may be helpful to state level geologists and hydrologists.

Labels: caves, conferences, geology, valley and ridge, virginia, water resources

At the DGMR Blue Ridge / Valley & Ridge Symposium the other week, we had a productive hour-long brainstorming session about unresolved issues in the geology of those two physiographic provinces.

I only attended the Blue Ridge session, and with the blessing of the conference organizers, I'd like to share the results of our ruminations here:

Unresolved issues in Blue Ridge Geology

General Outcomes:
Participants would like to see this as a start to a new state geologic map for 2015.
Participants would like have follow-up workshops to discuss these issues in detail.
Participants would like to develop research needs proposals on these key topics.

Specific Issues:
1. Ramp separating the Cambro-Ordovician carbonates from the rock of the Blue Ridge
a. What is its position?
b. Is it "thick" skinned?
c. Can field work (alone) resolve this?
d. What is its age?
How might this question be resolved?
A. Detailed geologic mapping at 1:24,000 scale across the Blue Ridge / Valley and Ridge transition
B. Applying geophysical techniques such as
a. Seismic reflection
b. Gravity studies
c. Reprocessing old data and reassessing old interpretations
C. LIDAR

2. Working out the problems with the Blue Ridge nomenclature and unit and fault correlations
a. Issues with variability in terminology and correlations across Virginia and between Virginia and North Carolina.
b. Map distribution of faults along the Blue Ridge - need to correlate their timing and extent
How might this question be resolved?
A. Detailed geologic mapping at 1:24,000 scale along the Blue Ridge (N/S) and across the Blue Ridge (E/W).
B. Models for the Blue Ridge need to be presented, discussed and synthesized.
C. Developing a basement "tool kit" for Late Proterozoic rocks: Since we can't afford to date every rock, there is a need for key characteristics for each unit to be agreed upon and applied in the field.
D. Expunge archaic terms and agree upon an appropriate vocabulary.
E. Regular focus group meetings and field meetings are necessary to resolve these issues.

3. When did the Blue Ridge develop its topographic relief?
a. Timing?
b. Is it in a steady state?
c. What is the history of the various erosional surfaces throughout the BR's evolution?
d. How does this correlate with the other physiographic provinces?
How might this question be resolved?
A. Detailed geologic mapping at 1:24,000.
B. Utilize low temperature chronometers such as U/Th/He at a sufficiently detailed spatial scale.

4. Role of the Mesozoic system in the Blue Ridge story and geometry.
How might this question be resolved?
A. Detailed geologic mapping at 1:24,000.
B. Need more geophysical studies.
C. Examine the effects/association of these fractures with water
D. Examine the relationship between features and mineral resource deposits
E. More detailed fracture studies are needed
F. Compare and contrast the Mesozoic rifting with previous rifting cycles
G. A seismic risk assessment needs to be done.

5. Mylonites in the Blue Ridge
a. Triclinic deformation
b. Palinspastic restoration of the Blue Ridge in 3 dimensions.
c. Oblique convergence in Thornton Gap area.

6. Is there an unconformity between the Catoctin and the overlying sedimentary units?

7. New mica ages and their implications for deformation timing
a. Is this related to thrusting sequence?
b. Ages in the Swift Run and Weverton ages were likely the maximum temperature. Deeper rocks likely represent cooling ages.

8. Issue with the cover sequence west of the Blue Ridge - just where is the PreCambrian / Cambrian boundary?
a. Middle of the Chilhowee group?
b. Is all of the Chilhowee group Cambrian in age?
c. Is it in the Lynchburg formation?

9. Are there any Ediacaran fauna in any of the Neoproterozoic sediments?

10. There is a need to reconstruct paleoenvironments in the Blue Ridge
i.e. Mt. Rogers' glacial deposists, etc.

11. Paragneiss story
a. What is the time/temperature path these rocks took?
____________________________________________________

Again, I'd like to thank Amy, Matt, Laurie, and the rest of the dedicated staff of the Division of Geology and Mineral Resources for organizing the symposium and following up with this break-out session digest. Everyone I know who was there felt it was really useful. Facilitating discussions like these between geologic researchers is a key function of a robust state geology agency, and it chagrins me to think there will be less of this in future years due to the recent budget cuts.

What do you think of this list? Are these the most important issues in Blue Ridge geology? What would you add or refine?

Labels: blue ridge, conferences, geology, virginia

Some geology-oriented terms made the New York Times' annual rundown on buzzwords. It's noteworthy that two of the (non-geological) others on the list (futarchy and edupunk) were coined by Virginia professors.

Labels: news, teaching, virginia, words

A quick note for fellow Virginians: CNN reports on the new efforts by the Bush administration to drill for oil offshore from the Old Dominion.

Labels: environmental, news, oil, petroleum, politics, virginia

Christie asks: What are your top ten geological resolutions for the new year?


For me, the list would include:

1. visiting the Galapagos Islands
2. visiting the high Andes (Cotopaxi, Chimborazo), Ecuador
3. finding a cool outcrop of graded beds in the Martinsburg Formation (late Ordovician turbidites in the Shenandoah Valley of Virginia) that Rick Diecchio told me about last week
4. "walking on the Moho" in Gros Morne National Park, Newfoundland (late summer)
5. seeing Snowball rocks and Ediacarans on the Avalon Peninsula, Newfoundland (late summer)
6. visiting Egg Mountain paleontological site, Montana
7. joining my colleague Ken Rasmussen's field trip to the Culpeper Basin, a Triassic rift valley in northern Virginia
8. some cool trip next winter break (2009-10): perhaps Patagonia? Or Antarctica?

I've also got some big teaching resolutions:

1. Running a successful and robust Structural Geology course for George Mason University (spring semester).
2. Running a successful and innovation Environmental Geology course for NOVA (spring semester).
3. Running a successful and safe Regional Field Geology of the Northern Rocky Mountains course for NOVA (summer semester).
4. Preparing and running a successful and groundbreaking Honors Historical Geology course linked with English Literature 242 at NOVA, where the English professor and I will bridge the two subjects with readings of Lyell, Darwin, "A Pair of Blue Eyes," and others (fall semester).

On other topics:

1. Finish my M.S.S.E. degree (July)
2. Buy a house
3. Put together a series of geology 'vodcasts' on local geology
4. Write a few freelance articles
5. Publish one cartoon per month in EARTH
6. Prepping (cutting and polishing) a backlog of rock samples from all over the place
7. Successfully moving the geology department into our new building

Labels: canada, ecuador, geology, msse, newfoundland, nova, south america, teaching, travel, valley and ridge, virginia

This just in from the office of Virginia's governor, Tim Kaine:

"In higher education, our October actions reduced schools' 2009 base budgets by 5 to 7%. For 2010, I have increased the reductions to 15% for all schools, except the community colleges and Richard Bland, which will have the reduction level increased to 10%."

Especially in light of what I posted earlier today, this does not bode well.

Full text of the governor's remarks here.

Labels: economics, news, nova, politics, virginia

As I mentioned yesterday, the Virginia Department of Geology and Mineral Resources has an excellent rock garden outside their office in Charlottesville, displaying a diverse suite of large rock samples from across the state's five physiographic provinces.

Here's Rick Diecchio (George Mason University) providing a sense of scale for the rock garden:


Here's a few of the samples that caught my eye, with my shoe providing a sense of scale (size 12, specifically) in each image...

Aquia Formation sandstone with Turitella fossils (Paleocene); King George County:


Balls Bluff Siltstone with mudcracks (Triassic); Culpeper County:


Conococheague Formation collapse breccia (Cambrian); Augusta County:


Cranberry Gneiss (?) showing well-developed lineation (Mesoproterozoic); Grayson County:


Kyanite quartzite (probably Ordovician metamorphic age); Prince Edward County:


Fossil Sigillaria tree trunk from the Wise Formation (Pennsylvanian); Wise County:


Unakite, the state rock of Virginia according to some (Mesoproterozoic); Rockbridge County:


Here's a link to the PDF (1.82 MB) with all the details about all the rocks in the garden, an impressive achievement just like the symposium.

Labels: conferences, fossils, igneous, meetings, metamorphism, primary structures, sediment, virginia

I spent this past Friday (12/12) at a symposium put on by the Virginia Division of Geology and Mineral Resources, in Charlottesville, Virginia. This excellent branch of our state government just had their budget disproportionately slashed, but they aren't letting it bring them down. In fact, they're breaking new ground in their unparalleled service to the geological public. In honor of the groundbreaking Virginia geologist Tom Gathright (who was in attendance), they organized a day of scholarship and conversation about recent advances in the geology of the Blue Ridge and Valley & Ridge physiographic provinces.

Rick Diecchio (of George Mason University) and I drove down together, getting up at the unholy hour of 5am in order to get there on time. Once there, we trundled past their excellent outdoor rock garden (about which I will post tomorrow), and inside to join the gaggle of more than a hundred geologists from the USGS, Virginia universities and community colleges, transportation agencies, environmental agencies, and the DGMR itself.

The morning session consisted of a series of talks about the Blue Ridge. We heard from Bob Millici (USGS), Scott Southworth (USGS), Chuck Bailey (W&M), Mark Carter (DGMR), Bill Henika (Virginia Tech), and Karen Rice (USGS). I won't post any of the juicy data details we heard, for fear of spilling any unpublished beans, but there was some cool stuff we learned about. There was also a poster session in the well-appointed library. Pete Berquist (Thomas Nelson Community College) and I had lunch out in the rock garden, where I chatted with three undergrads from our alma mater.

The afternoon session was given over the the Valley & Ridge province. We heard from Scott Eaton (JMU), Steve Whitmeyer (JMU), Dave Weary (USGS), Randy Orndorff (USGS), Joel Maynard (Virginia Department of Environmental Quality), and Wil Orndorff (Virginia Department of Conservation & Recreation, Division of Natural Heritage).

My favorite part of the day, though was a break-out session to discuss unresolved issues. There were three break-out groups: one for water issues, one for the Valley & Ridge, and one for the Blue Ridge. I went to the Blue Ridge one, and really enjoyed this unique setting. I mean, here I am in a room with a bunch of people who spend the majority of their professional time trying to understand how the Blue Ridge got put together, and we're just brainstorming together, thinking about big unknowns, big gaps in our understanding. The DGMR staff is compiling these results, and once they're distributed out to the participants, I'll post them here on NOVA Geoblog. We've been asked to share the results. Since there were two geobloggers in the room (me and Chuck), we reckoned that's a quick way to disseminate some of our ideas.

I'd like to thank the DGMR for putting on such a great meeting, in particular during such lean and uncertain times. The day was positive, affirming, and valuable on many levels. Readers, remember that you (yes, you) can still write to the governor and other state officials to protest the crippling 75% reduction in the DGMR staff.

Labels: blue ridge, conferences, geologists, meetings, valley and ridge, virginia

Yesterday, I took a three Honors students and a colleague to Difficult Run, Virginia. This is a hiking trail that goes from Georgetown Pike, in the tony neighborhood of McLean, Virginia, down through a deep, steep river valley to the Potomac River.

As noted a couple days ago, the trail is right across the Potomac River from my beloved Billy Goat Trail. In a recap from that post, here's a map of the area... Feel free to switch it to "satellite" view.



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

We began our trip by meeting up with Doug Dupin of the Palisades Museum of Prehistory, who joined us for our exploratory geohike. We walked a short distance down the trail and found a big (abandoned) quarry where it was rumored there was a good fault. This is one of these pieces of information that I heard somewhere, at some point. I couldn't find it in any literature, so maybe I heard it in discussion when I taught at George Mason University for a year between grad school and when I got my position at NOVA. Anyhow, I had never actually checked it out...

...So our first order of business was to review the criteria for identifying a fault: What would we look for? Fault breccia, fault gouge, slickensides, hydrous mineral veins, and of course, offset. However, here in the Virginia Piedmont, it's rare to have a good marker unit to compare on opposite sides of the fault: usually it's just schist on one side, schist on the other. In some places, you could add the presence of a fault scarp to that list, but being as how this was an old quarry, geomorphic features like that didn't seem likely. So our search focused on the search for fault breccia, fault gouge, veins of odd minerals, and slickensides.

A few minutes in, we found some slickensides on this boulder of float:

This is a boulder of migmatitic phyllonite, with a wavy texture due to mylonitic flow at depth. (The picture doesn't show this very well at all, though you can see faint undulations 'cascading' from the top of the photo towards the bottom. It's much clearer in cross-section.) Anyhow, the 'slicks' are a faint upper-left to lower-right lineation seen on this surface, one or two degrees off from the orientation of the ballpoint pen. The surface you're looking at here was a fault plane at some point in its history. Ballpoint pen for scale.

We did eventually locate the fault, uphill from this boulder. It was characterized by a zone of fault gouge (pulverized rock), three inches wide to a foot wide in places, and highly oxidized (presumably by oxygen-rich meteoric waters percolating along this fractured surface)... but there were no good marker units to judge the total offset.

Here's a different section through a similar rock (though I wouldn't apply the "phyllonite" textural description to this one). Instead of looking at the plane of foliation here, we're looking at a surface which is perpendicular to the foliation plane(s)....

Here in this image, you can see two cleavages... One which runs roughly upper-left to lower-right through the photo, defined by gneissic banding including bands of granite (light-colored; late Ordovician in age... Taconian Orogeny). A second cleavage runs roughly left-to-right through this photo. This second cleavage overprints the first. The overall interpretation is that the first cleavage developed due to lower-left-to-upper-right compression, forming the foliation defined by alternating bands of different compositions of minerals in an upper-left to lower-right direction. The second cleavage formed due to compressive stress sub-parallel to the pre-existing foliation, deforming it into a series of tight folds. The limbs of these folds line up parallel to one another, defining the second-generation, overprinting cleavage. Can anyone else add to this interpretation? Dime for scale.

Along Difficult Run itself, the outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. As I noted earlier this week, the area shows some nice exposures of granite pegmatites (keys, and the edge of the Pazzaglia volume, for scale):


On our field trip yesterday, we took at closer look at these beautiful pegmatites, and the associated amphibolite bodies. Take a look at this close-up... Dime for scale.

What's going on here? You've got a beautiful (euhedral/subhedral) example of an orthoclase feldspar ("potassium feldspar") crystal amid a bunch of quartz. But look closer at the feldspar crystal... this sucker has been fractured in many places, and it's shot through with very small veins of quartz. Somehow, as this pegmatite dike was cooling, the earlier-crystallizing feldspar was broken and intruded by the presumably-still-fluid silica-rich magma. Anybody able to expand on this interpretation and shed some light on how this all played out? Or contradict it and give a different story to explain this relationship?

In the neighboring amphibolite, we checked out these cool ridges of resistant rock which are centered on thin fractures. Here, you see a couple of intersecting joint sets, each of which was the "plumbing system" for silica-rich hydrothermal fluids (my interpretation). These silica-rich hydrothermal fluids impregnated the surrounding amphibolite with quartz, which made the immediately-adjacent areas more silica-rich, and hence more resistant to weathering and erosion: Hence, now that they've made it to the surface, they're weathering out in high-relief. Dime for scale.


A bit further downstream, Doug showed us a 'cave' (central dark area, just to the right of the waterfall) between the bedrock and a big slab of sloughed-off migmatitic metagraywacke:

We each edged into the 'cave' to the end, where Doug has shown that a distinctly-rectangularly shaped hole admits a direct beam of sunlight during the fall and spring equinoxes. From the inside, it's a striking arrangement, enough to make you wonder whether it's anthropogenic. However, from the outside I was unconvinced that the hole's position was anything other than natural. Doug's initial intepretation of the site was strongly influenced by the fact that there are some unambiguous petroglyphs a short distance away from here, and based on this proximity, I think it's acceptable to infer that Native Americans may have visited this cave. However, I interpreted the opening to be completely natural, with no need to invoke anthropogenic modification in any way.

We hiked on along a ridge overlooking Mather Gorge, sighting a fox and an accipiter (Coopers? Sharp-shinned?) and a few vultures, and returned to the parking lot as the sun dipped low in the sky. On the way back to campus, Honors students Ana and Hope fed us Swiss cookies and cheese & crackers. Altogether, it was a pretty great way to spend a November afternoon...

Labels: birdies, faults, field trips, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

This week, I'm taking some of my Honors students to Difficult Run, Virginia.

It's right across the Potomac River from my beloved Billy Goat Trail. Here's a map of the area:



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

Here's a look at Difficult Run, looking upstream from below one of the several waterfalls there:



These outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. We're going to look for a fault reported to be there, as well as the incision geomorphology of Difficult Run itself, and some nice exposures of granite pegmatites (keys for scale):





This field trip is less a guided tour, and more of an exploration, so I hope when we get back, I'll have some photos of new and interesting things to share.

Labels: faults, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

Chuck Bailey of William and Mary has started a geo-blog. Check it out at:
http://www.wm.edu/blogs/wmblogs/chuckbailey/index.php

Labels: blogs, geology, virginia

Starting this month and going into January, Arlington County, Virginia is hosting a series of free workshops designed to help citizens make more environmentally-sustainable choices in their homes and workplaces.


For more information, click here.

Labels: environmental, meetings, nova, virginia

Yesterday was the Geological Society of Washington's fall field trip. A group of about twenty of us went down to George Washington Birthplace National Monument, a stretch of land in the Virginia Coastal Plain, about an hour east of Fredericksburg. The trip was lead by Wayne Newell of the USGS in Reston and Rijk Morawe of the National Park Service.

Here's a map of the Monument, adjacent to a small bay formed as the valley of Popes Creek flooded with post-glacial sea-level rise (essentially the story of the entire Chesapeake Bay in miniature):


Wayne and Rijk are studying the coastal processes here in an attempt to use the Popes Creek as an analogue for Chesapeake Bay processes in general. One of the reasons they really like it is because unlike other small bays in the area, it has a spit (almost a baymouth bar) protecting it from the ravages of the tidewater Potomac (which it flows into). Here's the spit heading southeast across the mouth of Popes Creek Bay:


This rotted old wooden seawall was erected along the coast in the 1960s. This is on the Potomac, just upstream from the Popes Creek Bay. Effectively, this seawall serves as a "before" line, a marker which conveys the shoreline's former position. You can see how much erosion has taken place since then:


I'm less interested in these coastal dynamics, though, than I am in the bedrock geology. There were some bluffs along the river which exposed the Miocene Calvert Formation (clay-rich lower unit) topped by a foot-thick diamictite unit, and then well-rounded river gravels on top of that:


Here's Merily (sp?) from AGI checking out the sequence of strata:


My favorite part of the trip was looking at the variety of cobbles on the beach. These cobbles are derived from all of the mid-Atlantic's physiographic provinces within the Potomac River's watershed (Valley & Ridge, Blue Ridge, Culpeper Basin, Piedmont, Coastal Plain). All those physiographic provinces have been weathered to produce the sediment that the Coastal Plain is made of. In spite of their diminutive size, they give insights into the geologic history of Virginia over the past billion years. So if you're familiar with Virginia geology, you will see some familiar rocks here.

For instance, there were a lot of these Skolithos-bearing quartzite cobbles. These are pieces of the Antietam Formation, a meta-quartz-sandstone that crops out in the Blue Ridge province, many many miles upstream:


Skolithos is the name given to vertically-oriented cylindrical burrow trace fossils, which start showing up in the Cambrian period of geologic time, indicating the evolution of vascularized bodies among animals. They are usually interpreted as worm burrows. This cobble shows several different diameters of Skolithos tubes:


Here's a cobble of another distinctive Blue Ridge rock. This amygdular meta-basalt is a piece of the Catoctin Formation, a sequence of (mainly) mafic lava flows that erupted as the supercontinent Rodinia was breaking up in the Neoproterozoic era of geologic time. The white spots you see are amygdules: vesicles that have been filled in by mineral deposits. When lava erupts, it degasses. If the lava cools into extrusive igneous rock before the bubbles have a chance to pop, little round holes are preserved in the rock, like Swiss cheese. We call these "vesicles." When vesicles get filled in with deposits of minerals (from groundwater passing through the rock), they are called "amygdules," from the Latin for "almond," which I guess they resemble in an ellipsoidal sort of way:

(I showcased a very similar cobble here in March of this year.) Like the Antietam Formation cobbles, this Catoctin Formation cobble originated in the Blue Ridge province, and has tumbled dozens of miles downstream to end up out here on the Coastal Plain.

Here's one from even further away! This is a cobble of flint from one of the limestone units out in the Shenandoah Valley, the easternmost valley of the Valley & Ridge province. (I've previously posted on those rocks, too.) While the limestone which originally hosted this flint nodule has weathered away, the flint is microcrystalline silica: very hard, very chemically stable. It's a common cobble to find surviving out here in the Coastal Plain:

We also found some rocks that are distinctive occupants of the Culpeper Basin, a Triassic-Jurassic rift valley upstream. Here's a chunk of the Manassas Sandstone Formation, another rock that has been previously mentioned on this blog:


The rock I spend most of my time thinking about is the metagraywacke of the Mather Gorge Formation. (For one mention on NOVA Geoblog, click here.) Here's a piece of it that looks identical to the rocks you'll see near Chain Bridge, DC, or along the Billy Goat Trail (Potomac, Maryland):

This rock was metamorphosed ~460 million years ago, in the late Ordovician, although the original sediments are older than that: perhaps Cambrian or late Neoproterozoic in depositional age. This sample even had a little bit of hydrothermal quartz stuck to it, a common feature of Piedmont metamorphics...

Having covered clasts derived from the Valley and Ridge province, the Blue Ridge province, the Culpeper Basin sub-province, and the Piedmont province, there's nothing left in the Potomac River watershed except for the Coastal Plain itself. And sure enough, we saw Coastal Plain clasts too. Here's a chunk of the Calvert Formation that GSW Field Trip Chair Bill Burton found: He cracked it open and found a shark tooth fossil inside:

This is the first time I've ever seen a tooth preserved as a carbon film. Except it wasn't really just a film, it was more a three-dimensional external mold with a carbon film, and little nuggets of carbonaceous material rattling around inside. Shark's teeth are pretty common in Miocene deposits on the Coastal Plain, including C. megalodon teeth, but this style of preservation was pretty novel for me. If you're into fossil collecting, don't go to George Washington Birthplace National Monument, because collecting isn't allowed there. However, nearby Westmoreland State Park offers legal fossil collecting opportunities. It's about ten minutes further south.

I'd like to thank the field trip leaders and Bill Burton for organizing the trip. I enjoyed the excursion!

Labels: coastal plain, field trips, flint, fossils, gsw, rivers, sediment, virginia

Much of the geoblogosphere has been posting about trees over the past couple of days.

I'll just make a brief contribution here, showcasing some of the incredible fall foliage seen down near Konnarock, Virginia, while my students and I were down at the Virginia Geological Field Conference a few weeks back:

Labels: plants, virginia

As has been mentioned elsewhere, Virginia governor Tim Kaine made some big budget cuts last week. The Commonwealth of Virginia has a rule that they must have a balanced budget every year. So with the economy in such shabby shape and Virginia's income predicted to have significant shortfalls, Governor Kaine has decided to trim the Commonwealth's budget. The official revenue projections forecast a shortfall of $973.6 million for fiscal year 2009 and $1.54 billion for fiscal year 2010, or just over $2.5 billion for both years. According to a press release from Richmond, "Governor Kaine will balance the FY 2009 budget through state agency savings and spending reductions of over $348 million and additional steps, including a withdrawal of about $400 million from the Revenue Stabilization Fund."

Two entities in the state government that are getting hit particularly hard by the proposed changes are (1) the Division of Geology and Mineral Resources and (2) higher education.

(1) As I mentioned earlier in the week, this was an issue of much discussion at the Virginia Geological Field Conference last weekend. I would like to share here an excerpt from an e-mail I got after the conference from Chuck Bailey (W&M), the president of the VGFC:

Unfortunately, with Virginia's looming budget crisis, the State is planning to severely cut if not eliminate the Division of Geology and Mineral Resources (DGMR). Here are some of the planned cuts:

• 9 (out of a staff of 21) will be laid off
• 1 staff member will be transferred to the Division of Oil and Gas
• 4 staff members will be reassigned to support the Abandoned Mine Land project
• DGMR will be left with a staff of 4 on state-funded positions (of which 3 are currently supervisory) and will not, in any substantive way, be able to serve the Commonwealth. Details of the plan are on pages 14-15 of the Governor's budget reduction plan.

We have an obligation to fight these cuts with vigor. DGMR has served the Commonwealth well and needs to be maintained, even through the lean times. For
me it is clear that these cuts are a deliberate action to eliminate DGMR; consider the fact that within the Department of Mines, Minerals & Energy, of which DGMR is one of six divisions, the only layoffs are being incurred by DGMR.

Not only are these cuts are extremely shortsighted, but inherently unfair.
What can be done about this?

The most important decision maker who is likely to consider input from DGMR customers is the Secretary of Commerce and Trade. He needs to know how people use DGMR products/services, especially if they use them to make money or protect people and property, and why DGMR is important to the Commonwealth. Company letterhead is preferable. He is:

Patrick O. Gottschalk
Secretary of Commerce and Trade
P.O. Box 1475
Richmond, VA 23218

The Acting Director of the Department of Mines, Minerals and Energy (which includes the DGMR) is:

Benny R. Wampler, Acting Director
Department of Mines, Minerals and Energy
P.O. Drawer 900
Big Stone Gap, VA 24219

A letter to the Governor can't hurt either:

Governor Timothy Kaine
Patrick Henry Building, 3rd Floor
1111 East Broad Street
Richmond, VA 23219

People should contact their own Delegates and Senators.

A Virginia Geological Field Conference Yahoo! listserv has been set up to facilitate discussion for those who wish (search "thevgfc"). [Note: I would encourage you to read this discussion, as it points out that the total savings are pretty meager (~$10,000 for the upcoming fiscal year, because of severance pay and what-not) considering the crippling cut in services. -CB]

We need to act quickly and with forceful clarity on this matter.

Thanks,
Chuck Bailey
President, Virginia Geological Field Conference

Please take the time to write a letter to one or more of these officials to let them know what you think of the proposed cuts. Also, I'd like to give a shout-out to Lee Allison, state geologist of Arizona, who posted on this issue earlier today.

(2) The second major area where budget cuts are hurting this blogger is in the 5% cuts to higher education in the Commonwealth. Though I utilize the maps and studies produced by the DGMR, their budget cuts don't effect my paycheck. But when the Virginia Community College System has to slash its budgets by 5%, that does change my bank account balance. NOVA faculty and staff got an e-mail from our president last Thursday (10/9), informing us that though the College would continue to provide its services essentially uninterrupted with a 5% cut, faculty salary increases, scheduled for November, would be "delayed until July of next year." This is a real bummer, though for me personally the bright side of it is that I got my promotion before all this went down, so at least I secured that pay raise before things went sour. Just the same, I'm going to miss the extra cash that was 'promised' on the contract I signed at the beginning of the academic year. With everything getting more expensive, it's a tough on faculty when their salaries don't keep up with inflation.

So it's looking kind of grim in the Commonwealth, folks. While I don't think a letter-writing campaign will effect the higher education cuts much, the DGMR is a small entity that has gotten hit disproportionately hard. If you can write a letter to help save the DGMR, please do. It's an important state agency that does great work. Thanks!

Labels: economics, nova, virginia

On the way back up from the VGFC this weekend, we briefly detoured off the interstate (81) to go up Route 11, and across a singular natural feature in Virginia: Natural Bridge. This is a span of limestone going over a creek (and because it spans a watercourse, it is thus not an arch, but a bridge).

Unfortunately, this is all we saw of it:


The bridge is privately owned, and it's fenced off from view from Route 11, in spite of the fact that the road actually goes over the bridge. So we drove across it, but we couldn't really tell. And we didn't feel like stopping and paying the $$ to get in to see it from underneath.

In spite of that disappointment, what's pretty cool about the area is that it shows up well in this Google Maps "terrain" view:


Kind of wild: a natural bridge that's actually used as a bridge...

Labels: caves, limestone, valley and ridge, virginia

Yesterday, I mentioned that the main point of this weekend's field trip was to attend the Virginia Geological Field Conference in Marion, Virginia.

We arrived on Friday night at Hungry Mother State Park, and got some background information and logistical direction from the trip's leaders and the various officers of the VGFC. We also got some sobering news about how Virginia budget cuts will affect the Division of Geology and Mineral Resources... but more on that tomorrow.

On Saturday morning, we headed out to examine the geology of the Pulaski and Saltville thrust blocks, two of the slices of Paleozoic sediments that got shoved bodily northwestward during the Alleghenian phase of Appalachian mountain-building. The point of the trip was to examine the structure and stratigraphy of these two thrust sheets, in an attempt to compare and contrast them. Both are an example of "thin-skinned" tectonics, where sedimentary strata are deformed (folded/faulted), but they are disconnected from the tougher underlying "basement" rocks (the crystalline rocks of the North American continent beneath). Sliding along a big basal fault called a decollement, these sheets of sedimentary rocks created the northwestern fringe of the Appalachian mountain belt; a zone called the "fold and thrust belt." (This is in contrast to the "thick-skinned" style of deformation exemplified by the Blue Ridge province immediately to the east, in which the basement rock is itself deformed, and shoved up on top of these younger sedimentary strata.)

Here's two of the three field trip leaders: Loren Raymond (holding map) and Bill Whitlock (talking into the microphone), giving us relevant details for our first field stop:


Fred Webb (the third trip leader) used the same technique of large graphics as an aid in explaining the local geology. Here, he explores the geology of Saltville, VA, from a scenic overlook:


Here's Fred and Loren using another visual prop to illuminate the distribution of sediment types (Knox dolomite versus Moshiem limestone) on a farm in the Rich Valley:

Does anyone else out there use large visual aides like these on field trips? I think it's a pretty good idea.

There were a lot of people who attended the conference: over 120! Here's the crowd at the Saltville Overlook stop:


...and the throngs of geologists shutting down traffic on the way to another stop:


...and still more geologists all over the right-of-way at our final stop of the day:

Kudos to the trip organizers for coming up with a coherent way of running the trip with so many participants!

So why were we there? ...To look at these deformed sedimentary strata, and increase our understanding of the deformation mechanisms that accomodated strain during Appalachian mountain-building. Here's a look at the Max Meadows tectonic breccia, a zone of crumbled rock at the base of the Pulaski Fault:


Just above the breccia, the rock is still pretty deformed. Here's some intense folding and boudinage in dolostone & shale layers:


At another location, Honors student Hope W. shows a fault in the Nolichucky limestone:


In other places, folds were the main variety of strain observed in the rocks. Here, we see this in the Honaker dolomite (with elbow for scale):


Ditto for this exposure of the (Cambrian) Nolichucky limestone (enthusiastic caver for scale):


After a superb lunch put on by a church group, we strolled out in some karstic fields in the Rich Valley. Here, several field trip participants drop down into a sinkhole:


I was interested to see that there were a lot of Mississippian-aged evaporite deposits in this corner of Virginia. Saltville's salt was from the Maccrady Formation, as is this gypsum (note fingernail scratch mark):


Here's the spectacular final outcrop of the day, where we looked at deformation within the Cambrian-aged Nolichucky and Honaker Formations, as well as the Mississippian-aged Maccrady Formation they override at this location on the Saltville Thrust Fault:


Of note to you environmental types out there: Saltville was not only the "salt capital of the Conferderacy," but it was also the site of the very first Superfund site (due to dumping of mercury as a byproduct of soda ash + chlorine production).


And I'll just conclude the photo section of the post with a couple of photos of cool spiders we saw. Each of these arachnids is a good three inches in length (including legs):

I think the upper one is a 'garden spider.' The bottom one is silver! I've never seen a silver spider before...


All in all, it was a good day in the field. We returned pleasantly tired and hungry, and had dinner at the Hungry Mother State Park "The Restaurant". Over food, we discussed the pros and cons of field trips like this, and slept well that night.

I was particularly pleased to meet up with and hang out with folks like Cy Galvin (part of my pre-GSW dinner group), Jon Tso (Radford University), Pete Berquist (Thomas Nelson Community College), Amy Gilmer (Virginia Division of Geology and Mineral Resources), and Chuck Bailey (College of William and Mary). Pete, Amy, Chuck, and I are all W&M geology department alumni. Chuck mentioned the good news that he will soon be joining the geoblogosphere too -- watch this site for an announcement of his (surely to be excellent) geology blog as soon as it goes live.

Labels: arthropods, conferences, field trips, nova, stratigraphy, structure, valley and ridge, virginia

This weekend, I spent three days on an extended field trip down to southwestern Virginia with NOVA adjunct geology instructor Chris Khourey and four of my Honors students. We left Annandale on Friday morning, and made our first stop at Willis Mountain, Virginia, site of one of the most productive kyanite mines in the world.

Here's a Google Map of the mountain:


The Kyanite Mining Corporation was very gracious in hosting us. I'd particularly like to thank Mike Morris, who took two hours out of his day to show us the site and the mining operation.

Why mine kyanite? It's used as a refractory mineral: that is, one that won't melt under high temperatures. A lot of their kyanite is heated in kilns to produce a second mineral, mullite. The mullite is even more stable than kyanite in high temperature refractory situations. (It won't melt until it hits over 1800 degrees C!) Additionally, they cleverly saw up big blocks into dimensional stone for countertops and the like.

The kyanite mined at Willis Mountain is in a quartzite which also includes a fair amount of pyrite and hematite. We heard about the different procedures used to extract the non-kyanite minerals so that their end product is relatively pure and of constant quality.

Here's Mike showing the overall anticlinal shape of the deposit:

It's a plunging anticline, as you can probably make out from the Google Map terrain view up top.

Some of the dimensional stone, which I think is pretty spectacular:


Close up of the kyanite (light blue, on left) in the dimensional stone.


Nearby Baker Mountain also hosts kyanite deposits, which show a deeper blue color (Mike wasn't sure why, but suggested that chromium may be responsible):


Inside a huge storage building where the mullite (white powder at our feet) is stored:


Atop Willis Mountain itself, showing the weathered kyanite quartzite exposed there:


Honors students ask questions of Mike:


Mike and Chris standing near some fresh boulders of kyanite quartzite:


It wasn't all metamorphism and mining... I also noticed these nice raindrop impressions in a drying mud puddle:


After lunch atop the mountain, we hopped back in the van and hightailed it for southwestern Virginia, on our way to the Virginia Geological Field Conference. More on that tomorrow.

Thanks again to Mike and the good folks at the Kyanite Mining Corporation for hosting our visit!

Labels: conferences, economics, field trips, metamorphism, minerals, nova, piedmont, teaching, virginia

FYI, all you DC-metro-area-geophysicist types:

The October meeting of the Potomac Geophysical Society will be held October 16th at the Fort Myer Officers' Club in Arlington, Virginia in the Campaign Room. This month's talk will be: Infrasonic studies in the atmosphere using gravity wave models and a study of the 1988 PEPCON chemical explosion, by Dr. David Norris of Applied Physical Sciences.

Reception at 6:30. Dinner at 7:30. Talk at 8:30 PM. Allow 15 minutes for security entering Ft. Myer as all civilian vehicles are searched. To ensure access to and from Fort Myer use the Hatfield Gate. If you wish to attend dinner ($25), please make reservations with Joydeep Bhattacharyya at 703-284-1219 or via e-mail at jbhattac@bbn.com. If you wish, please feel free to attend the talk without dinner. Non-members and guests are welcome. Visit the PGS web site for new meeting announcements, etc.

Labels: meetings, pgs, virginia

Last weekend, I went out to the Virginia countryside to harvest some fruit. Several of my college friends have farms out there, with organic fruit free for the picking each fall.

Just thought I would share a couple of images...

Here's my cat Lola checking out a tarp full of ugly but delicious apples:

Last year, I harvested a huge number of apples and pressed them into cider, and fermented 8 gallons to make hard cider, which was pretty good. Or at least drinkable, if not exactly "good." This year, the orchard at Smithfield Farm was far less fruitful, so I only managed 3 gallons of cider.

The good news is that Orange Springs Farm hosts a mature pear tree that was heavily laden with fruit, and I collected a good sixty pounds or so of pears. Once these ripen, I'll turn them into cider too:


The harvest bounty matches up nicely with our recent cool weather. Fall is my favorite season, and now the leaves are starting to senesce and the nights are crisp and cool. Good sleeping weather. On a field trip this weekend up to Shenandoah National Park, the sumac and the Virginia creeper had already turned scarlet, and the deciduous trees won't be far behind. Happy autumn, everyone!

Labels: lola, plants, travel, virginia

A colleague mentioned this website to me yesterday: Voting America.

Maintained by the Digital Scholarship Lab at the University of Richmond (Virginia), the website makes us of so-called "cinematic maps" which show how geopolitical data have changed over time for the lower 48 United States.

The different maps tell stories over time; I think it's a cool display of interesting information. However, a major omission is the exclusion of Alaska and Hawaii as contributing U.S. states. Check it out; let me know what you think.

Thanks Judith G. for alerting me to it!

Labels: maps, politics, virginia, websites

The largest meteorite (or maybe comet?... we don't really know which) impact crater in the United States is in Virginia, underneath the lower Chesapeake Bay. In the Eocene, a large bolide (unidentified space chunk) slammed into the Earth. Dating of microfossils found in the same sedimentary layers as impact ejecta have provided a date of ~35.5 Ma for the event. The impactor hit on the continental shelf offshore of Eocene Virginia, carving through the Atlantic-deposited sediments there and gouging into the crystalline bedrock beneath (igneous and metamorphic rocks like the modern Piedmont province, but buried beneath Coastal Plain layers).

The crater was discovered over a ten-year process that began with offshore sampling near Atlantic City, New Jersey in the mid-1980s. Those drill cores came up with a layer of ejecta (including shocked quartz and little beads of glass called tektites) among the late Eocene layers of sediments. Searching around, eventually the crater was seismically imaged by oil exploration in the Chesapeake Bay in the mid-1990s.

Centered on Cape Charles, Virginia, the crater is about 50 miles across, but appears wider as sedimentary layers adjacent to the hold have slumped inward along listric faults. The James, York, and Rappahannock Rivers all trend into this depression, and ultimately the crater is probably responsible for the Susquehanna River taking on its southerly course. When sea level rose and flooded the valley of the Susquehanna, the Chesapeake Bay was formed.

A similar impact structure offshore of New Jersey, the Toms Canyon Impact Crater, may have formed at the same time as the impactor broke into pieces before impacting.

The lead-off image to this post is by the team at the U-Haul trucking company, which performs a terrific public service by finding out interesting things about the different states (and Canadian provinces) and posting them on the sides of their trucks with eye-catching graphics. A great many of the topics they choose are about geology, from minerals to fossils to impact craters to cartography and canyons. A while ago, I wrote an article for Geotimes looking at their program.

More information on the crater:

Wikipedia's entry on the crater.
W&M Geology Department's page about the crater.
USGS team examining the crater.
National Geographic article (2001).

Labels: art, cenozoic, coastal plain, geology, meteors, new jersey, piedmont, rivers, virginia

CNN reports on cougar sightings in the town of Blackstone, Virginia, a bit southwest of Richmond. The official line goes that since mountain lions (Felis concolor) were wiped out along the eastern seaboard in the early 1900s, they haven't been found anywhere except for a relict population in the Florida Everglades (where they are called "panthers"). But this one little town in the Virginia Piedmont has had more than the average number of sightings. I think it would be great if mountain lions reestablished themselves in the hills of the Old Dominion. Our deer population is out of control, and while it's unsettling to not be at the top of the food chain, ecosystem coherence takes a higher priority in my mind. Along similar lines, in 2004 it was reported that coyotes had moved back into Rock Creek Park, the large national park that runs through the heart of northwest Washington, DC. Park officials have suggested they wouldn't be surprised if black bears moved back in too.

Labels: critters, dc, florida, mammals, virginia

Another upcoming event that may be of interest to DC-area readers of this blog:

Labels: astronomy, evolution, teaching, virginia

I was sad to learn today of the passing of my first geology professor, and the man who got me interested in structural geology. Starting in 1963, Bruce Goodwin taught for many years at William and Mary, and in the fall of 1992, his Physical Geology class was literally the first course (8am on Mondays, Wednesdays, and Fridays) I took in college. Though I started out planning to major in biology, this course triggered an interest in other aspects of the natural sciences, which eventually led me to geology as a full time passion. Dr. Goodwin's style of teaching struck me as highly effective, and I still use some of his analogies (and jokes!) in my teaching today. His upper-level structural geology class capped off my undergraduate experience, and planted an interest that would eventually (many years later, in 2002) lead me to graduate studies in structure at the University of Maryland. Dr. Goodwin retired from teaching the same year that I graduated from the William and Mary geology department, 1996. I'm sad to think of him having passed from our world, and I wish his family the best. Here was a man who made lasting contributions to Virginia geology, and inspired 33 years of William and Mary geology majors.

Dr. Goodwin's obituary in the Daily Press.

Labels: geologists, virginia

A month ago, it was announced that there was an enormous stromatolite head discovered at a limestone quarry near Roanoke, Virginia.

It's two tons in weight, and over five feet across.

Being as I was out of town, I hadn't heard about it, but one of my Snowball Earth students alerted me to it on Wednesday evening.

Pretty cool! Read more here.

Labels: stromatolites, valley and ridge, virginia

Here's a few more photos from the recent field trip to the Massanutten Synclinorium in the northern Shenandoah Valley, Virginia.

Some more Arthrophycus (?) trace fossils in the Massanutten Formation:

Outcrop of the Massanutten Formation on Route 678, south of Waterlick, VA. Note that the bedding is dipping to the south (reflecting the overall "canoe"-shape to the structure of the Massanutten Synclinorium... this is the "bow" of the canoe...):

Shelly horizon in the Mahantango Formation. Mainly brachiopod debris, but also crinoid columnals:

Cross-bedding in the Martinsburg Formation's Bouma sequences. This is a sample I collected on Saturday. I sawed it open on Monday, then polished it and gave it a coat of clear acrylic. Sample length is about 5 cm:

Ditto. As above, we can see clear cross-bedding here, reflecting current flow in these ancient turbidites:

Bedding / cleavage relationships expressed at an instructive outcrop in the parking lot of a pet store north of Front Royal, Virginia. Bedding is clearly visible running subhorizontally across the picture, but the rock breaks vertically: a tectonically-induced cleavage:

You could hardly ask for a better outcrop to teach bedding / cleavage relationships. Here's a medium-sized anticline in the same outcrop (note quarter, center, for scale). It clearly displays a fan of cleavage orientations. Lovely!

Lastly, on that same note, here's a sample I collected fromthat locality, with bedding planes and cleavage planes highlighted through the magic of CorelDraw. The stripes you see on the face of the sample are formed by the intersection of bedding and cleavage planes, shown schematically in red:

Labels: appalachians, field trips, fossils, primary structures, valley and ridge, virginia

While toodling along the web on some other business this week, I stumbled across this publication by the Virginia Department of Mines, Minerals, and Energy.

I had no idea that there were any diamond finds in Virginia. But apparently there are, scattered across three different physiographic provinces!

On Thursday's excursion, Chris and I tried to find the "Front Royal Peridotite," one of seven locations mentioned in the DMME publication. It's a single dike which crosses State Road 626 southeast of Waterlick, Virginia. But to no avail! There were no outcrops visible on either side of the road, and there was a dense little cluster of houses bearing manicured lawns. Bummer. That would have been cool.

I'll try and visit a couple other localities mentioned in the report over the next year or so, and hopefully I'll find some of these igneous source rocks, though I don't hold out much hope of actual diamonds.

Labels: blue ridge, diamonds, minerals, piedmont, valley and ridge, virginia

The Culpeper Basin is a Mesozoic (Triassic/Jurassic) rift valley in northern Virginia.

As Pangea was breaking apart, a series of normal-fault-bound basins stretched open in an NW-SE direction (giving them long axes that run NE-SW). Some of them connected together in a NE-SW direction, and kept spreading further and further open. Through continued seafloor spreading, these became the Atlantic Ocean basin. Some did not keep opening, and essentially filled in with dirt. Those are the ones that are still preserved up on the North American continent today, including the Culpeper Basin. These basins vary in size, but they run up and down the coast of eastern North America, from Newfoundland down at least into the Carolinas (presumably there are more buried beneath Coastal Plain layers even further south than that). Collectively, these basins are referred to as the Newark Supergroup. They are characterized by immature sedimentary rocks and mafic igneous rocks.

Here's an E-W cross section through the Culpeper Basin, by Chuck Bailey at W&M:

Structurally, then, the basin is a graben, bounded east and west by normal faults.

The igneous rocks in the Culpeper Basin are mostly diabase, but there are some basalt flows too. The sedimentary rocks are a motley mix, including arkose, red siltstones, and lake deposits including siltstones and anoxic black shales. Along the eastern and western boundary faults, we also find coarser sediments that have been lithified into conglomerates. Sediments flowed into the basin from source areas both to the east and west, so you would expect the conglomerates along each edge to look a little different. Indeed, they do!

A modern analogue for the Culpeper Basin is the Afar Triangle region of northeastern Africa (Ethiopia, Eritrea, and Djibouti). Note the sedimentary influx from both the east and the west. Note the lakes, and note the mafic extrusions:

Back to the Old Dominion: I've mentioned the Culpeper Basin's eastern boundary fault before, back in March, when I posted this picture of the conglomerate that outcrops in Clifton, Virgina. It is characterized by lots of clasts of highly-foliated metamorphic rocks (derived from the neighboring Piedmont).

...But I haven't talked about the western boundary fault much. And since I visited it yesterday, today's the day to talk about it.

One of these western Culpeper Basin conglomerates is kind of famous. It's the Leesburg Conglomerate, and it outcrops near Leesburg. It's mostly limestone cobbles and gravel, with some quartzite, too, set in a red matrix. It's a beautiful rock. Here's a couple of field photos taken on Route 15, a mile or two north of Leesburg proper:

The Leesburg Conglomerate was used in the awesome columns in the U.S. Capitol's Hall of Statuary (topped by the much less interesting Carrara Marble of Italy).

Yesterday, NOVA adjunct geology instructor Chris Khourey headed out to Thoroughfare Gap (see map below) to check on a couple of field sites. Thoroughfare Gap is a water gap in the eastern limb of the Blue Ridge Anticlinorium, and it's also the western boundary of the Culpeper Basin. Both Interstate 66 and Route 55 pass through this striking landscape feature:

Note how different this looks as compared to the Leesburg Conglomerate. One thing that immediately jumps out at you when you see an outcrop of it is the large proportion of the cobbles that are pieces of the Catoctin Formation basalt (see more photos of the Catoctin in Monday's post on rocks of Shenandoah National Park). Here's a couple of close-up shots of such cobbles, bearing distinctive amygdules (filled-in vesicles):

But there's also plenty of limestone cobbles and gravel in there too, as this photo shows:

As with the Leesburg Conglomerate, the Waterfall Conglomerate's limestone inclusions are likely coming from the Cambrian & Ordovician carbonates exposed today in the Shenandoah Valley and other valleys of the Valley and Ridge province. More on that later this weekend, when I'll post some shots from the Massanutten Synclinorium.

Labels: basalt, culpeper basin, limestone, mesozoic, sediment, virginia

This weekend, I took a backpacking trip in Shenandoah National Park. Thought I would share a few photos today: scenery first, geology second...

Here's the view looking east from Skyline Drive:


The temperature difference due to elevation was striking. It was still early spring up on the top of the mountains, on Skyline Drive:


...But down below, it was green and lush (and sodden with pollen!):


I camped out for two nights near Corbin Cabin, and did a day-hike around Thorofare Mountain on Saturday, visiting this waterfall at lunchtime:


The geology of Shenandoah National Park is interesting: it records the assembly of the early supercontinent Rodinia at about a billion years ago, and then the breakup of Rodinia about 600 million years ago. The first event recorded is the generation of granite gneisses and granites due to the Grenville Orogeny. The oldest unit in the park is the 1.1 Ga Pedlar Formation, a granite gneiss. There's a slightly younger granite which intrudes it called the Old Rag Granite (~1.0 Ga), but I didn't see any outcrops (or float blocks) of it, so I'll not mention it further. There's a thin, patchy sedimentary cover called the Swift Run Formation deposited directly atop the granite gneiss and granite, providing a nonconformity surface. Atop that is a series of volumnious tholeiitic basalt flows: these mafic extrusions record the breakup of Rodinia and the opening of a new ocean basin: the Iapetus. In many places in the park, you can see "feeder dikes" of the Catoctin cutting through the older plutonic and metaplutonic rocks (see image below). There are also some sedimentary rocks layered atop the Catoctin (the Chilhowee Group), recording the transgression of the Sauk Sea on the North American platform. But I didn't encounter any good outcrops (or float blocks) of them on this trip, so I'll stick to the tectonic story: the Pedlar Formation shows us Rodinia getting put together, and the Catoctin Formation shows us Rodinia breaking apart. Later metamorphism due to Appalachian mountain-building resulted in changes in both of these rocks (development of "blue quartz" in the Pedlar, and the Catoctin metamorphosed to greenstone).

Here's a massive dike (possibly a "feeder dike" feeding surface lava flows) of the Catoctin basalt cutting through the Pedlar Formation granite gneiss, just north of the Marys Rock Tunnel. Note the columnar jointing extending perpendicular to the walls of the dike:


Having covered all that, I now propose to spend the rest of this blog post showing you the variety of cobbles and boulders in my campsite. I camped at the little wedge of land above the confluence of two streams. One stream's catchment basin was Catoctin, and the other drained outcrops of Pedlar. As a result, the "float" in my camp was all either Pedlar Formation or Catoctin Formation. I'll just run through them one after another so you get a sense of the range of variety in each formation.

You'll notice that the Pedlar is sometimes coarse, sometimes fine, sometimes well foliated, sometimes not so much. You'll also notice that the Catoctin varies a lot in terms of its extrusive texture: sometimes aphanitic (fine-grained), sometimes amygdular (formerly vesicular), sometimes it even runs to volcanic breccia. All of these original lithologies have been metamorphosed to various degrees in the Catoctin, which here can be seen by comparing the amount of green in the rock. This green comes from two metamorphic minerals: chlorite and epidote. Enjoy!

Pedlar Formation:



















Catoctin Formation:

Labels: appalachians, basalt, blue ridge, granite, iapetus, metamorphism, national parks, primary structures, proterozoic, shenandoah, virginia

Last week on one of the many field excursions, I found a nice cobble of amygdular basalt. Amygdules are vesicles (bubbles in degassing lava that didn't get the chance to pop before the lava solidified into igneous rock) that have been filled in with mineral deposits. In the mid-Atlantic, most amygdules are found in the Neoproterozoic lava flows of the Catoctin Formation, from which my cobble was presumably derived. The amygdules are typically filled in with zeolites, quartz, and jasper. This one doesn't show any jasper, but the basalt still appears to be basalt, too -- whereas the Catoctin typically is metamorphosed to greenstone / greenschist. I've noticed an association between jaspery amygdules and epidote formation in the metaingeous rock.

As with Skolithos-bearing Antietam Formation quartzite cobbles, clasts of the Catoctin deposited in the river gravels atop the Piedmont/Coastal Plain unconformity indicate a Blue Ridge provenance for the cobbles, and therefore a eastward-flowing river to deposit them 100 million years ago.

I took the cobble back to the lab and sliced it open on the rock saw. The brown circle in the background is a penny for scale.

Here's what the sawn surfaces look like after I sanded them down a bit and then scanned them:

Right purty, ain't it?

Labels: basalt, blue ridge, coastal plain, primary structures, sediment, virginia

Lastly, in this final picture, you can see (on the left and in the foreground) what a lot of the large bodies of migmatite looks like: mostly granite with wisps of mafic residue strung out as thoroughly-foliated xenoliths. Their common alignment is oriented in the same direction as regional foliation. This granite yields U/Pb ages of ~460 Ma, which is Taconian in age.

Labels: appalachians, migmatite, piedmont, virginia

Last Friday, I had a fun local field trip, in search of ultramafic rocks included in the Piedmont's metamorphosed accretionary wedge complex. My companions on the trip were David and John, both of whom are retired gentlemen pursuing geology as a hobby. Because they're doing geology for fun, they are among the most dedicated and interested students I've met at NOVA. Friday's trip was something I've been meaning to do for a while, and both of them thought it sounded like an eye-opener, so they came along too.

Our goal was to find some new outcrops that we hadn't seen before. Of primary interest were several mafic and ultramafic bodies included in the larger metasedimentary complex of rocks that we know today as the Piedmont. As I've mentioned before, these Piedmont rocks are interpreted as being the rocks of an ancient (Neoproterozoic - Paleozoic) ocean basin. When the ocean basin closed during Appalachian mountain-building, the sediments of the ocean got squished and squeezed between North America and Africa. Mixed in with them were chunks of the ancient Iapetus Ocean crust, which would probably be recognizable as ophiolites if it weren't for that pesky regional metamorphism they endured as a result of the collision. Up and down the east coast, there are outcrops of these mafics and ultramafics along the presumed "suture" zone between ancestral North America and terranes (blocks of crust) that were once a volcanic island arc in the Iapetus Ocean. As with most geology field trips, we also found some other stuff worth noting, even though it wasn't our primary objective.

Our first stop (located thanks to Diecchio & Gottfried (2004) in USGS Circular 1264) was in Clifton, Virginia, where we went to see the unconformity between the Piedmont metamorphic rocks and the Triassic sedimentary rocks which overlie them in an ancient rift valley called the Culpeper Basin. Tragically, instead of a beautiful outcrop, we found freshly graded surfaces and several new McMansions. There was only a small strip of undeveloped land, about 20 feet wide and 50 feet long which had any rock left. But in that area, we found an outcrop of soapstone. Here, John scratches the soapstone (talc) with his fingernail. It's soft!

In this case, the soapstone is interpreted as being metamorphosed ultramafic rock. Close to it, we found this piece of conglomerate:

The conglomerate is the base of the sedimentary sequence in the Culpeper Basin: it's the Reston Member of the Manassas Sandstone Formation. Notice that it contains clasts of foliated metamorphic rocks -- these were derived from the older Piedmont rocks it unconformably overlies. The Piedmont rocks got metamorphosed during Appalachian mountain-building, and then when Pangea broke up, the Culpeper Basin (one of the Newark Supergroup basins) opened up and got filled in. The source for the infilling sediment was the neighboring area, not surprisingly including pieces of the Piedmont. Up-sequence, the conglomerate is overlain by the regular Manassas Sandstone, which is a rich brick red in color (classic Triassic red beds), and contains a wealth of primary sedimentary structures. I found this one piece, which unfortunately broke into chunks when I picked it up:

It displays ripple marks, raindrop impressions, and a few horizontal branching trace fossils. Anyhow, that was about it for the Clifton stop. We were bummed about the development destroying the outcrop. On to the next location, Indian Run, on the east side of Annandale. There, using the geologic map that accompanied Drake & Lyttle (1981), we walked along the creek bed looking for exposures of rock. We didn't have to go far before seeing some heavily-rusted green rocks:

The above photo is dominantly chlorite, but check this out:

Pyroxene-rich inclusions (xenoliths? olistoliths?) were observable in the heavily-weathered exposures. The outcrops here were saprolitic, meaning they were essentially "rotten rock." David was struck by how soft they were. He said "It feels like velvet!" We turned our attention to the more coherent specimens which were weathered out and deposited as cobbles in the streambed. I got a watermelon-sized specimen that's about 40% massive peridotite and 60% greenschist. (I showcased this leprechaun-colored specimen last night in Historical Geology lecture, when we were discussing the Taconian Orogeny.) We also found intriguing hints of mountain-building in clasts like this:

That's a couple of beautiful folds in gneissic metamorphic foliation. As above, the bright green minerals are chlorite. We also found some cobbles of sedimentary rocks mixed in with the locally-derived metamorphic rocks. For instance, here's a nice semispherical cobble of flint, likely derived from the flint-bearing limestones of the Shenandoah Valley:

How did this flint nodule travel ~50 miles from its source area to its current resting place in Indian Run? Likely, it was transported by an ancestral version of the Potomac River, which brought many westward-derived cobbles eastward during the Cretaceous. About 100 million years ago, this river deposited a layer of cobbles all over our local area, preserved today as the Potomac Formation. It unconformably overlies the Piedmont rocks, and can be found today as the basal layer of the Coastal Plain. It's even found as a layer topping our highest local hills. The exposures in Indian Run actually offered a nice view of the unconformity surface, with foliated metamorphic rocks below, and unlithified Cretaceous gravel deposits on top:

Just to close out this post, I'll show a few other cobbles found in the streams. Here's a gneiss containing big, beautiful porphyroblasts of garnet:

And here's a Skolithos-bearing boulder of the Antietam Formation (quartz sandstone / quartzite), which I originally posted a few days ago, but is so gorgeous it should be shown again if I'm talking about boulders.

Finally, as a preview of tomorrow's post, I'll show a boulder which hints at the complex relationship between the foliated metamorphic rocks (gneisses) of the Piedmont and felsic igneous rocks (granites) which were derived from the partial melting of the gneiss. In other words, this is a boulder of migmatite: rock that has experienced partial melting. We'll explore this in more depth with some in situ photographs tomorrow.

Labels: appalachians, culpeper basin, field trips, flint, geology, nova, piedmont, unconformities, virginia

Picking up from yesterday's post about my hike along Windy Run in Arlington, Virginia...

Just downstream from the waterfall (and crossing the trail) is a recent rockslide. Between D.C. and Great Falls (12 miles upstream), the Potomac River flows through a canyon called the Potomac Gorge. It's hundreds of feet deep overall, and consists of a series of nested straths (bedrock "terraces"), each shaped roughly like (half) a canoe. (At the tip of each canoe is a waterfall leading up to the next strath). Where the vertical distance between straths is great, as it is at Windy Run, mass wasting events serve to break down the cliffs and reduce the crisp profile of the straths.


This rockslide happened in 2005, and the area of "raw" rock up at the top of the cliff reveals the source area for the rock debris below. I wish I had taken a photo of this three years ago when it was really fresh -- it would be an excellent place to do repeat photography to show how the talus pile and cliff face change over time. Upstream are several examples of older talus aprons that have been overgrown by plants and buried in soil. Already, you can see that a few Ailanthus trees (single, upright pole-looking things) have taken root in this fresh landscape.


Once you get down from the Windy Run trail to the Potomac Heritage trail, here's the view of the river, looking upstream. Virginia's on the left; D.C. on the right. A slight "shelf" can be seen on the Virginia side where a notch has been cut to host the George Washington Parkway.


As I hiked along, I found this dead mole. It's a big fat sucker, and it must be quite fresh: probably a casualty from the previous 24 hours. Lens cap is 5 cm in diameter.


More critter evidence: here's a couple of small tree trunks that were decapitated by a beaver. Again, this is recent -- note the fresh curls of wood shavings at the base of the trunk.


But enough with these living entities: let's look at some rocks. This is the metagraywacke rock that makes up most of the Piedmont in our area. This rock is metamorphosed to various degrees up and down the Potomac River, in some places all the way to gneiss and migmatite. In some places, it's schisty, but in others primary sedimentary structures are still preserved. Upstream by Great Falls, for instance, we find graded bedding in isolated less-metamorphosed, less-deformed areas. Down along this stretch of the river, it preserves a diversity of sedimentary clasts, as shown in this image:

Here, you're seeing the graywacke matrix mixed in with a bunch of dark chunks. Today, these dark chunks are mostly biotite, but that's metamorphic. Originally, they were probably mud clasts. Little pebbles of granite and vein quartz are mixed in too. It's worth noting that not only are they metamorphosed, but they're also stretched out in the same direction: foliated and lineated. Many are squashed into X>Y>Z ellipsoidal shapes (where the letters refer to the lengths of the different axes of the ellipsoid), like a mango seed. Lens cap is 5 cm in diameter.

Let's pause for a moment and bring people up to speed if you haven't previously spent any time thinking about Appalachian geology. These rocks are part of the Appalachian mountain belt, which runs from Newfoundland to Georgia (by one definition) or from Texas to Scandanavia (by a more inclusive definition). The Appalachian mountain belt consists of three provinces: from west to east: the Valley and Ridge, the Blue Ridge, and the Piedmont. Two of these are topographically mountainous today: the Valley and Ridge and the Blue Ridge, as their ridgey names imply. But the Piedmont certainly counts as part of the ensemble, and if you compare it to the other two, you'll find that it experienced the most metamorphism, the most deformation, and is intruded in many places with syn-orogenic granites (which neither of "the Ridges" can claim, at least not for Paleozoic orogenies). The Blue Ridge and the Valley and Ridge are deformed, yes, and even lightly metamorphosed, but the Piedmont is really where the action is: this is the center of the ancient Appalachian mountain range. These rocks experienced some serious continental convergence.

So what was the Piedmont before it was the Piedmont? An ocean basin. Before the Atlantic, before Pangea, there was an ocean basin off the "east" coast (it was really the south coast at that point, but no matter...). We call this dead ocean the Iapetus Ocean. The Iapetus was closed via subduction throughout the Paleozoic, and it closed for good when Africa rammed into North America, metamorphosing these rocks and raising the Appalachians. As subduction narrowed the Iapetus, sediments atop the oceanic crust were scraped off in a big jumbled pile called an accretionary wedge. (It is for this mixed-up melange that the infamous geo-blog carnival is named.) You want to see an accretionary wedge being scraped up today? Dive down to the Peru-Chile Trench, off the west coast of South America. You want to see a fresh one at the surface? Visit California's coast ranges, which are a Mesozoic accretionary wedge, raised above sea level. You want to see what an accretionary wedge looks like after it's been tectonically squeezed between two continents? Come to the Piedmont!

Our metamorphosed accretionary wedge consists of a bunch of the sediments that were deposited in the Iapetus Ocean, including what was originally graywacke (a mix of sand & mud). Occasionally, you find a sedimentary clast that's a bit more intriguing, like this one (white arrow):

What intrigues me about this little sedimentary cobble is the fact that it's foliated, which indicates metamorphism and differential pressure, but its foliation does not line up with Appalachian foliation. This cobble was foliated before it was deposited in the accretionary wedge. Therefore, it was derived from some area that had previously experienced mountain building & regional metamorphism (presumably a continent). That ancestral orogenic episode produced a source rock from which this cobble was derived. Then that cobble was deposited by sedimentary processes somewhere and (possibly later) incorporated into the accretionary wedge, which then was metamorphosed (& foliated) itself. Lens cap is 5 cm in diameter.

Here's another one, which shows its foliation a bit better:

When I see something like this, I start to wonder, where did this cobble come from? What was its sedimentary provenance? Is this a North American cobble that attained its foliation in the Grenville Orogeny (~1 Ga)? Is this an African cobble that got squeezed in some pre-Pangea Gondwanan orogeny? Is it derived from a nameless microcontinent that was formerly marooned in Iapetus oceanic crust (a la Madagascar) and is now accreted to some continent as an exotic terrane? Do the answers to these questions change how we think about the (1) closure of the Iapetus, (b) Appalachian Orogeny, (c) assembly of Pangea?

Elsewhere in the Potomac Gorge, there are other clasts in the accretionary wedge complex that encourage similar thoughts (for instance, you can check out the photos at the top of this page). Another question raised by these clasts is this: Does their position amidst such relatively fine grained sediments (the mud and sand of the graywacke) represent original deposition? Or is that simply tectonically-induced "shuffling" in the blender-like environment of the accretionary wedge? The rocks in an accretionary wedge are not stratigraphically coherent, but sometimes they have little areas that are. If these clasts are in their original depositional position relative to the graywacke matrix, what does that tell us? Are these landslide deposits? Or are these "Snowball Earth"-related glacial dropstones? Without the original sedimentary bedding (destroyed via orogenic metamorphosis & deformation), it's impossible to answer these questions, but it sure would be nice to know.

Lastly, I'll note that everything I've talked about so far (metagraywacke, mysterious clasts, quartz veins, granite intrusions, and regional foliation) are all cut by a series of joints, brittle fractures in the rock. These joints are arranged in a series of joint sets which intersect one another, resulting in the "blocky" nature to bedrock exposures in the Potomac Gorge (example). Here, along one Gorge-bounding cliff, I saw that the joints had begun to accomodate some sliding of the blocks of rock on either side. Technically, they aren't joints any longer, but faults, instead. Total offset is only a few inches, but it shows up well in a photo like this. Note the similar sense of motion on the more distant fault "scarp." A housekey (with pink ribbon attached!) is jammed into the closer fault to give a sense of scale.


All in all, an hour strolling along Windy Run provides some terrific opportunities for reflection on the checkered geologic past of the Piedmont and the Appalachians, and the continuing geomorphic evolution of the Potomac Gorge landscape. I enjoyed my little stroll. It was with reluctance that I turned around and headed back to the house to grade exams...

Labels: appalachians, faults, geology, mass wasting, plate tectonics, virginia

This past week, I stayed in Arlington, Virginia. My dad and stepmom were in London, and I was looking after my youngest siblings (both teenagers) by staying at dad's house and serving as the Responsible Adult. It's the same house I grew up in, and it has a lot of nice memories associated with it. At the end of the street, there's a little trail which leads off into the woods and downhill towards the Potomac River along a little creek called Windy Run. ("Windy" as in the weather, not as in sinuous, though it is that, too.) At the bottom, Windy Run launches off a waterfall and tumbles down into the Potomac Gorge. On Saturday morning, I decided to go take a hike down to Windy Run and reacquaint myself with the landscape and its rocks. Here's the view from the top of the waterfall looking across the river into D.C.


Here's a view of the waterfall from the side. The big ice-rimed log at the base is about a foot and a half in diameter, to give a sense of scale:

On the way down the trail, there lies a big boulder of quartzite. This is my first rock. By that, I mean that this specific boulder is the first time I learned to put a name to a chunk of the Earth: my dad taught me that it was quartz, and I committed the name to memory. Today I would note that it's milky quartz, indicating hydrothermal deposition. (Tiny inclusions of water in the crystal lattice scatter incoming light and make it appear white.) Its upper surface is covered in black lichen. Pondering it anew on Saturday, I wondered if learning the name of this boulder in the late 1970s was the first step leading to me towards my ultimate career as a geologist. Lens cap is 5 cm in diameter.


My "first rock" lies at the base of a hill, below a linear trail of other quartz boulders. This array likely represents a subterranean vein of hydrothermal quartz, a common feature in the Virginia Piedmont.


For instance, here's a big vein of hydrothermal quartz (center) cutting across the metagraywacke host rocks at the top of the Windy Run waterfall. It's about a foot wide, and emplaced at a ~20 degree angle to the regional foliation (which strikes ~N25E). The quartz vein is oriented approximately vertically, just east of true north.


Here's some more vein quartz in the metagraywacke matrix. Foliation runs approximately left-right across this image. Note how there are large bodies of milky quartz arrayed semi-parallel to foliation: these are probably best interpreted as boudins: the results when a tabular vein of quartz was broken into chunks, and these chunks were smeared out along along the foliation during mountain-building. Boudinage (the process of producing boudins) is a somewhat brittle behavior (breaking) and somewhat ductile (smearing): under the proper combination of high temperature and directed pressure, quartz can act like pizza dough. It's capable of being molded, but also capable of separating into coherent pieces. We call these "boudins" because they resemble sausages strung out in a row ("boudin" is French for sausage). Here, only one boudin is shown, but click here for some other examples. The boudin is about 3 cm in thickness, to give a sense of scale.

There are also smaller quartz-imbued veins (white arrows, extended with dashed lines) in this rock, cutting across foliation at nearly right angles. Note how the "infusion" of quartz along these thin fractures makes them more resistant to weathering (they stand up in high relief, as seen in the lower left). This set of small quartz veins was likely emplaced at the same time the rock was being squeezed during mountain building, for reasons I explain in the next photograph.

So here's my stress interpretation of this rock. The big blue arrows represent the principal stress direction. To simplify, you could think of one blue arrow as representing Africa and the other as North America, pushing on these poor oceanic sediments caught in the middle. The yellow arrows represent extension. As the rock gets compressed in from "top" to "bottom," it gets squished outwards left to right. This deforms pre-existing quartz veins by rotating them into parallelism with foliation, and also potentially boudinaging them into chunks like the big one. The green ellipse demonstrates this overall process. One way to accommodate the rock's stretching in the yellow-arrow direction is by opening up small fractures (like the ones on the left) which get infilled with quartz.


On my walk, I saw a couple of exposures of hydrothermal quartz that strained the definition: that is, they weren't all quartz. Instead, parts of them (~5%) appeared to be granite pegmatite. In this shot, you can see several large crystals of potassium feldspar set in the quartz. Large flakes of muscovite were also semi-common. Lens cap is 5 cm in diameter.


Here's another shot of the same phenomenon seen elsewhere on the trail: large crystals of potassium feldspar and muscovite set in the "quartz vein." At what point do we stop calling these quartz veins and start calling them pegmatite dikes? Is a single crystal of non-quartz enough to change our perception of the fluid from hot mineral-rich water to wet magma? Like many things in geology, these features indicate that phenomena like dikes and veins are on a spectrum between end-members. In other words, there are shades of grey in how these things form (in addition to how we interpret them). By the way, the greenish hue is algae, not epidote. Lens cap is 5 cm in diameter.


Granite dikes (including pegmatitic ones) are reasonably common in the Virginia Piedmont. Here, as a Windy Run example, is a small granite dike I saw in a boulder on my Saturday walk. Lens cap is 5 cm in diameter.


Tomorrow, I'll explore a rockslide I saw on Windy Run, as well as the nature of the metagreywacke itself. Stay tuned, rockhounds...

Labels: geology, granite, structure, virginia